Here's a way to make MK's endings not suck: just make them all canon. Plain and simple. I don't want to see what would have happened to Reptile if he just randomly decided, "I feel kind of treasonous today" and killed Shao Kahn all Brutus style, I want to know what happened to Reptile after Raiden defeated Shao Kahn or whoever is the canon winner of the game.
Maybe they intend 2 get rid of him (coz' he's such a sh!t character), so they just didn't bother putting any effort into his ending. Who knows; in MK10 he might be half Cyber-Sub Zero, Half Shao Khan, half Man Bear Pig.