specifically
onNPCAppear(NPC number) (e.g. onNPCAppear(1) would trigger every time a SMB3 Goomba appears on screen)
onNPCKill already exists and it could make coding simple boss battle changes (such as a LUA version of Murphmario's old Bullet Birdo) and flower power ups much simpler.
alternatively onNPCManifest(NPC number; SameSection boolean) (or however you would put it) so onNPCManifest(1, true) would be if a SMB3 goomba exists in the section a player is currently in, whereas onNPCManifest(30, false) would be if a hammer exists anywhere in the level, for example. Whichever works better.
there are probably 200 holes in this idea but I'm putting it out there anyway.
EDIT: btw, I'm not new, I just lost my old account details