This is something that came up in the "making permadeath matter more" thread, and I think that it's 100% needed. It also ties in well with some of the ideas from the thread about redesigning health, and health tanks, specifically.
The core idea here is this: there are some ways to improve your specific character (bonuses), and if that character dies then those bonuses are lost. The bonuses need to not be so huge that it's like going up a bunch of civ levels, and they need to not be so time consuming to collect that players will be inclined to save scum. Whatever the final design is needs to conform to this paragraph, but that's a lot of flexibility.
The first thing that comes to mind for me is Cave Story, which had an awesome three-level system for each weapon you used, and you'd collect EXP for that specific weapon in the form of these little triforce-looking shards that popped out of most enemies that you killed. Of course, this encouraged grinding enemies, but it also encouraged not getting hit. And grinding all the way back up to level 3 on a weapon gets progressively easier as you go, since your weapon power is increasing as you grind, which is satisfying all on its own.
It seems clear to me that whatever this bonus structure is, it really probably needs to be something that you can get by killing regular monsters. If EXP is going away, and mana potions/drops are going away, then it really would make a lot of sense to be having enemies dropping these little pieces of "bonus" for you, that get applied to you.
But I don't think that trying to apply that per spell, or even per spell color, is a very good idea. First of all that would require a lot of new HUD clutter, and the game just doesn't need that. But secondly, unlike Cave Story, here you can have lots of weapons equipped at once and it's not even clear which weapon you are "currently using" unless you just mean "the last offensive spell I used," which would be kind of arbitrary in a lot of battles especially with the proposed combo systems that reward you for switching between several spells in tandem.
Anyhow, really this is more about the CHARACTER than their weapons or equipment, anyway. This is something that gets applied to the character, and which makes you a little sadder to lose that character when the character dies. So the most obvious thing to apply this to, which several players already suggested in another thread, is to health.
Having your character have a low "base health" and then have some form of "magical shielding" or similar on top of the base health, which gets charged up (even leveled up, potentially) based on these enemy drops, could be interesting in a similar fashion to what was going on with Cave Story.
But I'm not sure it should end there, and there were some other really interesting ideas having to do with health capsules that fracture as you take damage, or things like heart pieces that could be combined into new health capsules that would later break, etc. This whole idea with the stacked bonuses seems like it could play very well with some system like that, but it seems like it could even go beyond it.
At the moment I need to run, but those are just some of my current thoughts jotted down. Would definitely love to hear what others think on this, as I think this is something that's needed to tie several elements together. See the flow chart in this thread for more info on how I think it fits in: http://www.arcengames.com/forums/index.php/topic,9534.0.html