At this point after some discussion we are looking for triggers that will fit each element thematically and are repeatable.
A bit of caution though try to be original.
Triggers...
they should depend on the opponent too?
example:
whenever a creature is burrowed (depends on both player)
whenever a creature you control is burrowed (depends on you, but maybe future cards can alter this a little)
And how hard should those triggers be to activate?
Brainstorming a little (these are all creature-based triggers, really easy to activate)
whenever a creature is mutated
whenever a creature is momentumed / creatures with momentum comes into play
whenever a creature produces
whenever a creature is quinted / immortal creatures comes into play
whenever a creature dives?
whenever a vampire creature damages the opponent?