I have preordered AVWW months ago, but haven't played past the intro mission since the infinitely respawning monsters completely killed any sense of accomplishment in combat. I have played the current version a few hours and it feels much better.
So, I have some thoughts from a perspective of a more or less new player about rewards for defeating enemies.
The problem with many of the "kill x of monster y" unlocks is that the only thing they unlock is a new enemy type, which is a good thing from a player perspective since it is more content, but bad from "in character" perspective, since new monsters are more difficult. It would be nice if all unlocks had an in-game positive aspect.
Mobs dropping consciousness shards is definitely bad idea, because for a new player they are nearly useless. Even after guardian powers unlock, most of the initial ones are not really directly useful, other than the wind shelter one, and in any case they are limited by the scrolls, which can only be obtained through missions, and the costs are so low that you will gain more shards in the course of the mission than you could use on the scrolls. A reward for killing mobs, if any, must be much more immediate to influence new players.
I'm not really sure if any rewards are necessary, health drop and just the fact the monster is no longer there seems mostly fine. Although, I think the respawn rate should be pushed back to something on the order of mission expiration. Right now I have seen monsters respawn outside a larger maze rooms. I don't think there is so much backtracking in AVWW that it is necessary to provide monsters to fight when doing it, and it would give a greater sense of "holding back the tide" as it were. And I suppose a small enchant charge drop would be nice (maybe for absorbing health orbs at full health, when those are left behind and just float somewhere I always feel momentary need to go and collect them despite the fact that it's impossible), as long as it is minor addition to containers and the game communicates that properly.