Actually, this is a terrific question.
In playing all the classic MK games all the way up to MK11, NRS/Midway has established a lengthy history of completely reinventing the proverbial wheel when it comes to Mortal Kombat games. There are literally three examples where NRS kept gameplay relatively intact from title to title. For all the other Mortal Kombat games, they've had the tendency to completely overhaul literally everything.
Mortal Kombat 2 is the first MK game that was a true legacy title. All but 2 of the original cast returned, and not only did they all retain their special moves (and their inputs), but they all got even more special moves. As a matter of fact, this game is literally the perfect sequel because it is an expansion of the previous title in every way - more characters, more moves, more finishers, more stages, more secrets, everything. Yet, it was created directly on the foundations of it predecessor.
The next example of a true MK legacy title is Mortal Kombat Trilogy. I consider Trilogy its own title separate from MK3/UMK3 because it was the first MK title to be released exclusively on home consoles (Playstation 1 and Nintendo 64 for you youngins out there) and its gameplay was completely different from MK3, even though it borrowed heavily from it. It kept the run mechanic, kept all the MK3 characters and their moves, but added more onto them. It even added a brand-new mechanic, the Aggressor.
And then the final example of a legacy based Mortal Kombat game is Mortal Kombat Deception - it kept the 3-stance system that was started in MK Deadly Alliance, along with the 3D-era sidestepping mechanics. Most of the returning characters kept the fighting stances/special moves that they had in the MKDA, but they also had many of their classic moves returned back to them. Sub-Zero got his Ice Clone, Raiden got his teleport and Superman, Scorpion got his teleport back (though it became a kick for some reason). Some of the returning characters weren't completely built on legacy; Scorpion had one of his fighting stances completely exchanged for an inferior one, and his weapon changed also.
Other than these three games, every other MK game has undergone a drastic overhaul when compared to the game before, and though it provides for some fond memories and excitement for a new game, it also elicits confusion for veteran players and does not allow for the development of a consistent MK meta for players to build on. Going back the example of MK9, anyone that began their foray into competitive fighting games with MK9 had to rewire their fighting game circuits in order to become proficient in MKX. Virtually none of the fighting game skills transferred over directly from one game to the next because their metas were too different to even compare.
DJT showed us all during his EVO MK9 finals match against REO the importance of patiently walking your way in towards a zoning opponent. Such a moment is nigh impossible to replicate in MKX, nor is it even necessary because the run button combined with ridiculous jump-in punches nearly nullify any kind of zoning in that game. Character walk/dash speeds were very important in MK9, but in MKX nearly everyone walked at the speed of continental drift because dashes and running were so powerful.
And now currently, any skills that players gained during the MKX era are useless in MK11. Meter is different, variations are completely different, there's no run button because walking/dash blocking replaced it, breakers were replaced with breakaways, the wakeup system is night-and-day different, thereby killing okizeme, etc...
I suppose that what I'm trying to say is this: the lack of legacy in NRS titles is in fact one of the most consistent pieces of legacy that NRS has offered for the Mortal Kombat games, whether it's to the benefit or the detriment of the games and players. Personally, one of the things I love the most about another franchise that I enjoy, Super Smash Brothers, is that no matter which one I play, the skills I gained in playing the previous titles will translate to the current and future titles. I know all of Donkey Kong's moves, all of Yoshi's moves, Bowser, Pikachu, Dark Samus, etc. But even though I know my characters moves and inputs and have a basic idea of how they will play before the next game come out, the differences in the engine and mechanics, plus some of the properties of the attacks themselves ensure that I never feel like I'm playing the same game as before.
None of the Smash games feel anything alike. Even among Smash 4 and Ultimate, which are the most similar, there are so many differences that the experience is completely different, even though Ultimate kept all of Smash 4's characters and mechanics. NRS can do the same thing with Mortal Kombat - build on legacy, while adding new things to make the games feel different. Casual players won't notice or care one way or the other - they buy and play the game because of its name. But us veteran players would likely enjoy the franchise much more if we didn't have to deal with the frustration of losing things that we liked and gaining unsuitable replacements, as we start completely from scratch from game to game.
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