Ahaaaaa, another feedback thread.
Okay, let's have a look here:
#1: That sounds like a fine idea, BUT, you'd have to make sure there was some incentive to use the units that were capable of it. Otherwise you'd basically just get a reaction of: "Well, why would I want to use this unit? He'll just bypass my mountain barrier!". Not to mention that the mountain walkers would have to be very strong, or they'd still simply be no threat against a walled off city/area. And typically normal, non siege human units arent meant to damage cities.
This though goes along with another issue that I've been aware of, which is: How do you give the player incentive to use the units with more requirements? Like the Norse archery ranges, it's easy for someone to plop those down and spam thrower types.... and that's it. I keep thinking there needs to be a bit more incentive to drop down some of the other things like wells and deer parks or whatever to get the units that want them, instead of just being content with letting these buildings put out just one unit type over and over, which I strongly suspect is what alot of players do. So.... yeah. I dont mean to bring up even MORE issues in the feedback thread, hah, but this suddenly reminded me that I'd been meaning to mention this one anyway. It seems important, particularly if you're doing things like giving some of them mountainwalk or whatever.
#2: Hm.... having mountains not affect line of sight normally doesnt entirely make sense to me. It's a bloody mountain; how does one see past it? I could see it INCREASING line of sight (and maybe range?) if something is on top of it, but.... yeah, not blocking line of sight is likely just going to seem illogical. I can see siege units firing OVER it though, maybe.
#3: That sounds fine. I assume units could overspend in that way as well, then? I know attacks work that way but I dunno how it is for movement.
#4: The soul idea sounds really good! Getting the human military units to fight is pretty much half the point of the game, so having a resource like this sounds like a great way to entice the player into doing it. Particularly when they're already at a score gate point. You could probably do alot of interesting things with this idea; not just with the myth units, but with other stuff, and it might open up even more once you guys get to the point of doing add-ons and such. It might indeed solve the problem of players using JUST myth units without any normal army nearby.
I would say, have varying amounts of "soul energy" or whatever depending on the unit. That bit about differentiating units might come in handy here. Harder to get units (AKA, the ones that require more resources or more types of resources, or both) could be worth more, enticing the player to try to use them when they can. These might also be ones that could get mountainwalk or other abilities, to differentiate their function.
And finally, about the issue of complexity:
While I agree that you need to be careful, and at least try to keep things somewhat accessible, I also think that there's only so far you can go with pure accessibility before it starts actually impeding on the design. You can really only worry about this one so much, particularly with a deep strategy title like this one. Chances are, alot of the players that are going to really be interested in this game are probably used to alot of complexity in their games as it is. I think this is one of the appeals of this one. And really, for all of it's parts, the game seriously is not that hard to learn. The only part people seem to get a bit stuck on is the production-goods, but that's fine. Everything else? Nobody really seems to have too much trouble with. But yeah, worrying about that one too much is only going to stifle things, and that'd be a shame. One way or another you're GOING to run into a point, probably sooner rather than later, where the complexity is going to go up no matter what. Particularly as you get into expansions, or bonus DLC, or whatever.
The trick isnt so much keeping complexity away.... the trick is just making sure that your mechanics make sense (which for the most part they currently do) and are explained well by the game. If you do this, and also explain them well in your patch notes, and if player questions continue to be promptly answered as so far they have been, then I dont think you'll have any issues. And in the end, the game will be better for it.
So, that's some of my thoughts on this.... hope it helps.