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?