There is a very roundabout way by using spawners using their limit property. You'll need to put one somewhere offscreen, ensure it's loaded and have it detected within your arena for its limit. once the enemy is killed, the spawner will be below its limit and spawn a new enemy, which you can use to trigger a contraption. (eg. goomba steps on noteblock, dispenses shell, hits switch, key drops from sky).
it's currently not on the list of things that could make it in the short term or long term as far as our plans go. This could of course always change depending on what features the site might offer to support this and/or how the priorities of the team may change.
