In all honesty, it'll never please everyone. I don't mean that in a "we're all a bunch of whiny spoiled kids" way; rather, I mean that MK is a series that is old enough to have problems when changing too much... But also being a problem if it stays too long the same. Am I making sense? Hopefully.
We're taking into consideration that the new era features only relevant new characters, but I would like to think that this is valid for previous games too. I don't think that they ever wanted to release a half-assed game with poor characters - but even so, they did. Taking that into consideration, I am afraid of what MK X may be. IMO D'Vorah and Ferra/Torr look to much like comics' characters, and while they may be nice I'm still struggling to see how exactly they fit the MK world. Cassie is OK and she represents elements that are part of the series since, like, ever. Kotal Kahn is interesting since he's a descendant of the friggin' Outworld Emperor and fan-favorite boss Shao Kahn, but I have mixed feelings about his existence. Of course this is all personal opinion but I have a feeling that this is exactly one of the issues when we talk about "new" characters in MK canon: MK4 actually got some (some) new characters right and they did fit the MK world, but after that how many characters are we able to pin down as fitting additions to the series? Seriously, we have a bunch of piss-poor imitations of previous characters, others are extremely generic and then we have a few that may not be imitations or really generic but just won't fit, like Nitara. And so far I'm seeing shades of this issue with the new MK X characters.
I totally agree that the series must move forward in order to be relevant and coexist with newer games! But I have to agree with Tarkatan in that familiar faces make a difference, especially in a series as old as Mortal Kombat. This isn't a spin-off, it's one of the main games - old characters end up making a big difference in the end. I don't think they should fill up half or more of the roster with old faces, but they definitely will have to keep some more names from the past in order to balance everything, otherwise some of you cheering up for MK's "ressurection" will end up being reminded of why exactly the 3D era didn't work until MK 9. This is the fine line that MK walks as a veteran series, by the way. As I said, if it changes too much too fast, it'll fail. If they stay in the past, it'll fail too. The key is in balancing everything until they're ready to move, and therein lies my fear: I really can't agree that the "ultimate massacre" from MK 9 was the way to do it.
PS.: Since I was "around" when Tekken 3 was released, let me say that it worked because Tekken was still fresh AND because they did exactly what I was speaking of previously: the new characters did fit the series' universe and/or had links to previous characters or elements from the series. You can bet that it would have totally failed if it was such a radical change like some MK games did. You just don't go "to hell with you fanbase, I'm going to kill all of your favorites and replace them with new names" and expect it to be a pleasant change. You gotta prepare your fans/players.