I know that part of what I'll say has already been said a few times, but here's my two cents :
In AVWW each player add to the team power, and monsters are tougher to compensate.
In skyward collapse, each player brings actions meaning even if the ressources are still the same, you can do much more each turn, even if its just placing ressource tokens or smiting tiles at first, later when ressources aren't a problem anymore, it adds A LOT of towns, units, etc... everything you do is basically doubled past the first 10/15 turns.
In most of your games, the multiplayer brings more power to the player and give some to the comp to balance things, or just let the player search for higher challenges and up the difficulty. I know I play in a higher difficulty most of the time when I'm with a friend than what I'd choose in singleplayer. Your game customisation and difficulty selection is well made and offer an experience fit for any type and number of player, use it!
In the end, it's not a bad thing, you keep the base gameplay but just add more players, in bionic dues, it would mean playing with multiple exo on the same screen instead of switching, like it seemed to be the case at first (I didn't play prior to release), that was the good choice, but no need to increase actions of the bots or limit players to one action only like I saw suggested here, just give one action to each player and then comp plays, if it's easier this way, so be it, there's higher difficulties to compensate when you launch the game.
That's just based on your previous games and I may be saying what was already there during alpha, but that's my opinion nonetheless.
That's for the general approach on multiplayer.
Now let's answer a few problems I saw in this topic :
Limiting exo's to 4 isn't a problem, as long as each player gets a choice in the matter. I think allowing each player to select an exo during game creation is the best answer here (1 each for 4 players, 1 each and 1 more for player 1 with 3 players and 2 each for 2 players). It means that you'd probably need a game setup screen like in AI war though. During missions with 2 or 3 players, each should be able to change exo with one of the unused ones like in single player or ask someone for a swap. With 4 players, only the swapping is left, sending a popup to ask a player like in most rpg/hack n slash when you ask for trade, only here it would swap both exos on agreeing, could need a lot of coop so as to choose with who to swap since many players could need it at the same time (which is a GOOD thing, you're supposed to play multiplayer for the COOP part). Another way to do it would be to keep control of your bot all the time, and just swapping the positions, not exo ownership, except when you swap for an unused bot with 2 or 3 players. It would give a "your problem is now mine" feel.
As for inventory, each player should be able to custom his own exo and shouldn't be able to touch others so someone isn't taking back items from another player's exo or tinkering with it while he's not looking. Which means you should be able to swap exo "ownership" even on the city map. As for sharing the items, well, it's basic hack n slash/rpg cooperation, if you keep playing with dickheads who steal all the loot, maybe you should just stop and start finding a real team, you play for the COOP like I said. Add a "roll" option if you must. Alternatively, you could lock items to the player who found it or give them at random and add a "trade" system between players, like in most hack n slash/rpg again.
I don't think any of this is necessary in a proper team but eh...
Anyway, on to the next point, mission choosing should be done by first player, with a possibility to "ask for a vote" in order to see what's the general opinion/needs (by placing markers on map?), but ultimately, player 1 should be the one choosing.
Now on the "hallway problem", I really don't see it. It's dumb but true. Sure it takes 15 tiles so 15 turns. But in the end, if there's no ennemy in sight, it barely takes a second to decide which way to go, forward! The only thing that would be needed would be either to end the turn automatically once each player made his move (hallway could be done in 15 to 30 second top this way) or to add an end turn shortcut (didn't check the options to see if there was one) to speed it up a little (double the previous solution time to consider the clicking/moving of the mouse otherwise)
All in all, it's really not that big a problem, since if there's a situation to assess (like an ennemy bot in the middle of the hallway) everybody should stop anyway to see what's the best way to deal with it.
The only problem would be with a proper turn based game where each player is separated, in this case, if player 3 is first in the line, it takes one more turn for player 1 and 2 to move since they're blocked by player 3 in the rare case where everyone is stuck to each other which is probably a VERY VERY BAD IDEA in this game, even MORE in a hallway situation, it really shouldn't happen for real, but let's say it does, in this case, a really simple thing to do would be to make the turns simultaneous for players, this way, everyone plays during the same "player turn" with a limit of one action, and then the comp takes its turn. Thus, the first in line move, then second, then 3rd and finally the last one (if you're playing with 4 players). It should barely take 5 seconds to do it and frankly, with the number of bots, I think you'll spend more time watching the computer move than waiting for your friends. And of course, swapping bots/positions count as an action for both swapping players while I'm on the "turn" topic.
The main argument I can see here is that everyone seems to think that people will do whatever they want, and in this case, yes, it's utter chaos and it will probably always end in the death of the team, but hey, that's NOT proper coop, so of course it takes a bit more time to play than solo, but that's compensated by the fact that if everyone plays right, you spend more time thinking but less time exploring/fighting since many player can do it at once, so in the end, hallway or not, a 20 minute mission in singleplayer should be more or less a 20 minute mission in multiplayer.
Of course, it goes in pair with the "what will my team do" since if someone launches a rocket while you were preparing to fire your laser, it disturbs you and your plans, but then again, it's COOP, I'll keep repeating that until it's heard, if you want chaos, you get chaos, if you want coop, you get coop, multiplayer is multiplayer, if it's not a successful experience, it's not the game's fault but the player's. If you want to play coop, the bare necessity is to at least talk or write to your friend so you can all check what can/must be done in any given situation. Moreover, most of the people who play online nowadays use teamspeak mumble or skype (just a few exemples) to speed up the discussing process by talking instead of writing, but hey, even without that, you can still chat in game if need be.
And you can always add an undo button like in skyward collapse, if you have a beef with someone, just don't use your action, undo the last and talk it out with the concerned player. As long as everyone didn't use their action, you should be safe anyway, the common "player turn" won't end.
As for the "support you friend from afar thing", the swapping should help, and if it doesn't, then you should have gone with him instead of wandering on your own, player's fault, not the game, solved. A roguelike is supposed to be unforgiving, blame yourself
There's lot of possibilities once you add 4 bots instead of one, like playing with an brawler supported by a sniper, or making a group of 4 ninjas to stealth kill all the bots of a room at once with careful positioning. Even if your friend is a few tiles away, with the range some weapons get, that's easy to cope with, and it will make bot customizing and item choice all the more important.
Of course it'll be a little different from the single player experience but like I said in the beginning, it's also the case in all the other games anyway, it's close, but not totally the same, it's not such a big deal, don't forget that you said the important was to make it fun, and it should be.
That should answer the main problems right now, if there's anything I'm missing (which is probably the case), feel free to tell me, also I'm not trying to judge or anything, everyone play the way he wants, but don't blame the game for a situation the player caused. It's all the more true in a roguelike. That's all I'm saying. As I see it, the multiplayer could be a blast and work real well. There's no reason it shouldn't. As long as someone doesn't go take a shower during the game... If players are focused on the game and speak with each other, the game could go quite fast, even faster than solo on the same difficulty due to the added power on the players side. In the end it's all a question of teamplay.
One last thing before I go, if it's really needed in the end, why not use propulsion to give one more stat, a move stat, each turn you can move several tiles, not just one. Obviously not 9 tiles in a turn, but 2 or 3 at first depending on your exo type and up to 4/5 with items, or just 2/3 all the way, it doesn't matter, you could speed up the moving phase like that, be it solo or multiplayer, and of course ennemy bots should be able to do the same, why not add new type of ennemies with faster movement for example?
It's not an ideal solution as it makes planning more difficult (and planning is everything in a roguelike) but it could probably be done and the game would still be enjoyable as long as it doesn't get you to move too far. It just save turns (and allow you to take cover, retreat faster, but that could be balanced by altering something else, like the number of covers on the map by making them more scarce and reducing the range of some weapons/ennemies to compensate the faster moving in range). The problem with this would be balance mainly so don't go that way unless you really have to. It would also probably make it a mix of tactical rpg instead of a pure roguelike.
tl:dr I'm with Teal_Blue on most of the things he said, and moving down the hallway isn't really a problem since it shouldn't take a normal person more than one or two seconds to move one tile forward goddammit!
With an auto end turn function when all players did their action, it should speed things up even during firefights and out of corridors
Sorry if it's really long, I tend to speak too much.
I'm open to critic so don't hesitate and fire away!
I'm all for multiplayer, I'll do what I can to help achieve it.
P.S. Just a thought about the pilots, 1 pilot per player would be OP, for example if someone takes an epic exo with genji (only one exo would be epic, the one this player chose), another one take emma and buy things, then a third one take meg and equip the parts and exo both others brought with them. Could be quite powerfull. But all relying on the same pilot while being in different exo doesn't make sense.
A good solution could be to allow one pilot per player but reduce their effect in multiplayer to a fraction of what it should be. Half for example. Emma would only get half of her price bonus, meg would only get one more level instead of two, genji would get epic bots who are only half better as their solo counterparts (who could be renamed legendary if need be), rey could revive once, tuck could only reveal loot an terminals in twice his sensor range instead of all the map and axis could stay the same but view missions from 5 tiles in solo instead of 4 which would be multiplayer. Just an idea, but it could balance multiple pilots while keeping things interesting in multiplayer. I would still make the game easier than having only one pilot but so is having multiple exo anyway, like I said, better let the difficulty selection be the judge here. Players can always crank it up
