The Alien stealth mode. Not sure if this is truly revolutionary in tabletop RPGs but it sure is damn fun and a reason for Colonial Marines to keep those motion trackers active until the energy runs out.
Stealth mode is a turn-based exploration system. The characters move carefully through an environment, scanning for enemies and surveying the spaces they pass through.
It’s actually substantially similar to the dungeoncrawling model we’ve seen since early Dungeons & Dragons, right down to the length of the turn. The innovation, though, is that after each player turn, the GM takes a turn to move enemies.