A couple of notes while I continue to mull:
1. I was definitely planning on ammo being tier-less, for all the reasons mentioned above.
2. If you want a good idea of the sort of general spread of rarity of ammo types, games like Half Life 2 or Unreal Tournament are mostly what I was thinking of.
3. The hybrid idea above was my favorite last night as well, cool to see that overall folks were thinking along the same lines. Though, like a lot of folks, I think that the final implementation needs some tuning there.
4. When it comes to cast times, I think that can be really interesting but also really annoying. The crests will allow you to charge up spells for extra power, though, so I think that whole range of options is pretty well already covered by when you choose to customize your spells with crests and a charge augment gem. Not that that's in the playable builds of the game yet, but I think that works better as an extra option through crests rather than as part of the base mechanics.
5. Having just three bars, with a neutral point between two elements, is something I'd considered a bit last night but it still would take up a lot of space on the screen and might be kind of confusing. And in terms of changing the overall nature of spells at different temperatures or whatever, that's kind of interesting in some ways, but honestly I think I'd rather just make those as separate spells so that players can plan in advance rather than having to really work the combo system in a heavy way to get certain spells. I've been thinking of any combo system as mostly being about DPS and cooldown times and other mathy things like that. Having a lot of extra bars on the screen just... feels wrong, to me, in a lot of ways. In others it's attractive, but it feels a lot more like a fighting game than the sort of game I'm going for at core here.
6. It's a good point on Indiana Jones not going to some temple to get a health kit. Then again, in a zombie movie they do go to a mall to get food and things of that nature. Dawn of the Dead, etc. In this game you're party Indy and part Survivor. That said, one thing I'm mulling is if I should just take out the Outfitter crafting workbench as well (there's hardly anything there) and make it so that you scavenge for things like snowsuits, etc. Those would be a lot more interesting and convincing of rewards, and when it comes to (say) wooden platforms, players are already getting those primarily from exploration rather than crafting. That would get us down to one main kind of crafting workbench, which I think would be a really positive change for new players (then later, as they get higher level, they start running into the crestsmith and the spellshaping gems).
7. Regarding ammo... this is making me think of some other things in other threads (the one about warp mechanics in particular), and some of my past work in my unreleased game Alden Ridge. Specifically, that hoarding and collecting ammo are both problems. And being able to leave a mission, get more SUPER AMMO and come back is also a problem. Will comment more in the other thread about the more general thoughts attached to this, but as regarding ammo my thought is: why not make it so that certain events in the game will give you back to "full stock" of each ammo type, but that that's the ONLY way to get the ammo of those types. Going back to town would be the obvious candidate, but that's really broken in and of itself.
Rather, my thought is that as the mission system is implemented where you have two missions to choose from, and completing one of them clears both of them and adds two new missions to choose from... that's the line in the sand right there. Each mission would give you a stock of ammo at the end for your major weapons, and you'd not get refreshed on that until you ended the next mission. So you could use that ammo on side quests or on the next mission or whatever, but you still only have so much of it and then it's gone. Which would be fine under the hybrid model since 80% of spells wouldn't use ammo at all anyhow, and it would really amp up the survival aspect of the game a bit, which is watered-down at the moment. Actually, thinking about the permadeath thread, you could even lose all your ammo when your character dies, which means that you then have to do without your super-weapons until you complete the next mission, which would be really interesting. On the lowest two difficulties that could be skipped, but for the top three action difficulties that seems like an interesting thing.
Will take some more mulling, then I'll probably write up a more detailed design for what I'm actually planning, with all the little changes and additions based on this thread. There are a few other related things I'm also thinking about and might write up, as I think that all of this should tie together cohesively (of course), and that's a lot to mentally juggle. But I really like where these threads are heading. Thanks for all the thoughts, folks, it's been a huge help and really enlightening.