yea! this is something we had awhile ago. we even had a whole sort of carnivore/herbivore predator/prey system where the carnivores ate the herbivores and the herbivores ate you. even so, we've tried more general things like you're talking about and it always sort of falls apart because they kill each other and just sitting patiently until they die is nearly always more preferable to interacting with the situation. there is so much danger (holes) all around that even having enemies get killed on spikes can make it so the challenge is removed. i think that's alright some of the time and we might add spikes killing enemies eventually anyhow since they're rare enough but yea...
so right now there are sort of three "classes"
enemies - don't hurt/interact with others, other than spatially, but do hurt/track you.
vineguard - these are the preplanted and planted-by-you weapons/abilities and etc. they can be interacted with by you AND enemies and hurt/heal/help both sides.
tumbleseed - interacts with everything.
so the emergence sort of comes out when enemies and things interact with preplanted weapons/abilities. for instance, if there's a poison laying out there, then an enemy rolls over it, it'll then be infected and possibly infect others, or it could turn invisible, shoot a shotgun, fill holes up all around, cause an avalanche etc.
that's sort of where we like to see that stuff since it can also hurt you, and the vineguard itself then isn't such a part of the player's world but more the space between the tumbleseed and its enemies which we like because the control scheme itself is indirect and you're really the vineguard (the vine you actually control), not the tumbleseed.
that all said, maybe we need to up the amount/quality of preplanted things in the world to get a bit more chaos and interest in the mix. would also help preview a lot of the powers out in the world...