Portal reloaded adds time portals ontop of the normal portals.
Splitgate is an FPS that adds portals on top of Halo style combat. Very fun but may be hard to find a game these days its kinda dead.
Age of empires 3 is an RTS that adds shipments. These shipments increase the pace of the early game and allow for way more personalized play as well as allowing players to react to different things without shifting their entire build order.
GTFO is a horror fps where you and 3 other human players take on raids and these raids are changed each season. When the raids are retired they are gone. You must beat the lvl 1 raids to unlock lvl 2 and so on.
Radio Commander. Its an RTS where you sit in a tent in Vietnam and give and recieve orders via radio. You have a map that you can mark where things are based on the info you get.