Fair enough, but I don't think ammo will fix the overarching problem with the current mana system.
The problems brought up were:
4. Mana is important for making there be an opportunity cost to using spells. If a spell is really powerful but also really expensive in mana, then you have to weigh that against weaker-but-cheaper spells. This is hugely important for balance of the player side of this game.
5. Mana is also important, like health, for limiting the length of expeditions and making it so that you have to return to settlements every so often to re-kit and prepare for a new expedition. There's a fine line, though, between returning too often and not often enough. Players should be self-sufficient in the field for a goodly while, but not forever. There's a lot of thematic and gameplay reasons for that, but I don't really wish to enumerate them all right at the moment (this is long enough already!).
Opportunity cost only exists when to get something, you have to give up something else. There's also the problem of limiting expedition length. For ammo to work well with these strictures, you have to have few ammo types with mana possible big spells (to make you give up one big spell for another), and then you have to have a total ammo limit that allows for you to be good in the field for awhile, but not forever. I take this to mean something like those newfangled core missions. So, in the core mission, you can fire X big spells, and have to choose between them. You have opportunity cost also in using them early or late in the mission, only on bosses, or saving health by killing a large mob with one. However, this means that you either have to farm for ammo outside of core missions (tedious and boring), or you get it handed back to you on a reload, which means you blast off your big spells all the time when you can go back to your magic ammo box for the side missions.
In any system, you can't have many types of ammo, and you can't make it tedious to re-arm, and you can't expect anything but the major bosses to have any challenge then. But even if you do overcome that, and make the opportunity cost viable for side-missions, the only thing you change is the big spells and you make the smaller spells even less interesting as they have no scarcity.
An ammo system I could get behind would be using the shards we already have in game (that drop all the time and you have thousands of). Big spells would take ridiculous amounts of shards, but would allow you to do massive damage very fast, whereas overall, the smaller, cheaper spells would be better if you could fire them over several minutes. That creates opportunity cost for all spells, and gives ammo that is abundant for cheap spells and scarce for big spells. It would also encourage killing enemies with the appropriate spells (as you can net bigger gains). Of course, this is also imminently farm-able, but it at least makes the farming somewhat interesting. The part of the problem it doesn't cover is that you never have any reason to go home, just to keep going and fighting. There's no need for recovery, just farming.
As always, the best thing I've heard of yet has been offering up some heat effects like those of plasma weapons in Halo, without unnecessary complexity adding in multiple heat types, instead offering up better ramifications for hitting your "limit". This covers opportunity cost (big spells will overheat you), but also takes out the need to go back to the settlement. That's why I suggested the overheating making you lose health, as that would give you reasons to go home and recover.
EDIT: Actually, looking over that first post on redesigning health and mana again, I saw the "mana tank" idea, and keeping mana the same as it is now. That would work pretty well too, though would be more bland than some of our other crazy ideas.
Poll