Time dilation might actually be a cool thing to explore in a Mass Effect game at some point.
Its obviously not a thing with FTL travel, but it could set up an interesting story if the first mission of a game had the protagonist confront the game's archvillain near a black hole where time dilation is an issue. The archvillain scores an early victory and is able to delay the protagonist there, and when the hero and his or her crew finally escape from the surface of the planet or asteroid or what have you, they emerge into a galaxy where years have passed during the days they spent stuck near the black hole, and all the archvillain's plans have succeeded.
Of course for that sort of game the villain's plans would have to be more mundane than the Reaper's, like seizing power instead of annihilating all life. Also being a space rebel could be fun. 