The idea would be to add an option on which Killer Neutrals could be set (by the map maker) to trigger on a determined number of turns.
Example:
-Player A, on round 2, takes Killer Neutral X (which is set to trigger in 4 rounds).
-On Round 6, Killer Neutral X would kill all armies in it (if still belonging to Player A). Reverting back to its neutral state.
This would create a series of vast possibilities in terms of creating maps, contexts and defining game-play.
Also, and this is very important, maybe they could even actually be a part of bonus while the clock-count goes down, before triggering.