Might as well chime in. Be warned, there is a novel.
Although the patches for Mortal Kombat XL paved the way to a cleaner game, I highly doubt that NetherRealm will build off its formula for MK11. Not because it is a good or bad concept to follow the blueprint for a previous product, but because NRS has the burning desire to make all their games different from one another, which isn't necessarily for the worse. You know the difference between MK9 and MKX is as clear as night and day, and it's no different with the Injustice games.
That said, here's what I'd like to see when it comes to gameplay.
1. I don't want the run mechanic to come back unless there is a consequence for using it to close distance, such as a recovery window when cancelling the run with the block button. As it is right now, it's essentially a "get-in-almost-for-free" card when used correctly. Instead, I'm in favor of giving walking more of a purpose, though not so much that it overshadows dashing entirely.
2. I doubt the variation system will come back, because after a while it started to seem like that moves just got plastered to specific variations just to keep up with the trend of assigning three variants to each character. If it does make a return, it might be interesting to see the number of variants reduced to two, or for each fighter to have different versions of specific moves in their arsenal (think the V-Triggers from SFV: Arcade Edition).
3. I know that this is probably beating a dead horse, but let's face it: anti-air options got worse and worse with each game ever since we left behind Mortal Kombat 9. Why am I seeing moves that take over ten frames to start up, be labeled as "anti-air attacks" in Injustice 2? If you're talking anticipation, then yes, but on reaction? Not really unless they don't opt to use their best aerial option for whatever reason. Anti-airs did improve over the course of MKX's lifespan, and they are better in Injustice 2 than they were in the prequel, but there are still inconsistencies that need to be straightened out.
4. Blockstun on certain blocked specials would feel so misleading when punishing them, especially moves that are between -10 and -15. This needs to be addressed. As an example, when I played Injustice 2, I found myself having difficulty punishing Robin's assassin dash (or whatever that move is called when he runs in and goes to the other side after hitting you). I often felt I was making a gamble trying to punish it and I'd be surprised if other players don't share this sentiment.
5a. This is probably going to be controversial, but fuck it. Can we please stop with the interactables? I didn't mind them so much in MKX because all required stamina to use, none were unblockable (that I recall), and the ones involving corner escapes could be punished on a read, but meterless invincibility (as far as super meter is concerned) was still a part of the equation and I can't say I'm a fan of that. As for Injustice 2...let me point you towards a familiar scenario involving the corner interactable in Atlantis. Do you need me to say any more?
5b. If, for whatever reason, NRS simply can't bear the idea of excluding interactables, then I will not be surprised if the old discussion of banning interactables starts up again. In which case, I suggest taking a page from the book of Injustice 2 by implementing a "tournament mode" that bans interactables from competitive matches. That way, both the competitive and casual players get what they want. This is just my personal outlook, but when I play matches, I'm interested in fighting the opponent, not the environment.
6. Less advancing strings that are safe and/or without gaps, if you please. If any are safe, I hope they leave the offender at negative frames, at least.
7. I didn't welcome the return of invincible wake-ups in Injustice 2 and I won't welcome them in MK11 either, unless they do not start combos and are unsafe, or if backdash is the only invincible wake-up option as long as the timing is on point. I realize that NRS tried to take that direction with Injustice 2, but they probably did not realize that the difference in the meter-burn systems for MK and Injustice would lead to exceptions with this rule (example: Deadshot's BF3). Hopefully they are more vigilant.
8. In MKXL, mix-up heavy characters had their options slowed down a bit to give us a better chance at blocking them (such as Kung Jin's F2 and Cassie's F3). I hope that this trend continues ,and I also hope that mix-up characters cannot make themselves safe on incorrect guesses and are punished hard in return. If a mix-up knocks down rather than launches, I'm fine with it being safe, but not at advantage on block.
9. I like the adjustments that were made to the armored moves in MKXL, and I hope NRS continues to follow this trend for MK11. That is to say, there cannot be any armored moves that double as combo starters and are coupled with plus frames, and if there are any, the player will need to come off more than one bar to have access to such an option. In addition, I hope backdashes see improvement so they can make a useful reversal option.
10. Less hitbox-specific scenarios, please? This tends to happen with female characters, but I confess that Scorpion had this benefit as well, in some instances.
There's more I can add, but I'll stop here for now.
