The main problem with your 'save it for MK10' argument is will the same rule also apply to MK10? What I mean is, are you also saying they need to drop members of the cast from MK10, just so they save some hype (as you call it) for MK11? It seems an odd way to wish for games and an even odder way for a games developer to behave, always leaving something out for the next potential next installment. Ideally as games consumers we want to wish for each new version of a game we love to be the best yet, not ask for them to deliberately hold something back, essentially not making it the best, just so we have something to look forward to next time......
As for new characters suck, does that not totally depend on what they are and how they are developed? Besides, characters like Jax and Kitana were 'new' once, do they not then also apply to 'new characters suck'? You also seem to now be suggesting that MK 1-3 characters shouldn't be in MK10, so wouldn't that then be all new characters, who you just said suck??