Hopefully nothing, Mortal Kombat is worse than most, though recent games have raised the bar for the series a lot.
Hmm, one thing they could learn is to stop milking their games. Street Fighter still does the stupid re-releases of each version, MvC was also doing it before Capcom fucked it up completely. Most anime fighters, including even the insanely popular Dragon Ball FighterZ, tend to be replaced entirely within a year, as many of them are made by the same developer. Not very enticing to buy a fighting game no one's gonna be playing a year from now. I mean, Mortal Kombat itself I feel releases games way too often, but at least they're spaced out by a few years and that alone creates a lot more interest.
Another issue with most fighting games when it comes to generating interest is that they're trying too hard to please hardcore audiences, in ways that alienate all casual players and sometimes even the hardcore fans too. I've seen people talking up how cool it is that Guilty Gear Strive's UI tells you when a move is unsafe or whatever, that stuff will not make anyone interested in your game other than a very select few.
But I think mostly it's how these games are released. I'm looking forward to Guilty Gear Strive, but then I think, why buy this game now for a price I can barely afford, when I can just wait a couple years and get a highly enhanced version with triple the content for a fraction of the price? That's the thing that turns people away I think, it's why Street Fighter V was so maligned. Mortal Kombat and Smash are the only ones not doing this - yeah they still have DLC characters, sure, but comparatively, the base game is pretty complete.
Edit: Tekken, right. That's another example of a game that's doing it right, and getting rewarded for it.