After I've bought MK Deception two days ago, I started gathering together some of my ideas for MK7. I am aware that this is the most unoriginal thread ever, and that people - specially the guys at Midway - don't give a damn at what their fans want or think, but I'll risk posting this anyway.

* Arcade Mode *

For starters I'll just say that I really like MK: Deception's Arcade Mode although I think it would be better if it was a little faster and more responsive. I'm not a big fighting games expert so I really can't get too technical with this, not to risk making a fool out of myself. Tekken still feels more fun to play than Mortal Kombat, but I would like to see Tekken with gushing blood and fatalities.
It also needs to be harder to finish Arcade mode. Deception is ridiculously easy when it comes to this, as Onaga is the only really hard to beat opponent until you get the hang of it. Bringing back Endurance battles would help.

* Konquest Mode *

This is where I would like to see the real changes. I'm a complete fan of adventure and role-playing games and I was really hyped up to play a disappointing Konquest mode.
I think for MK7 the Konquest mode could evolve into something really special. Many fans have been asking for a create a character mode, and Konquest would be the perfect opportunity to do it, if it was merged with some classic role-play/adventure features.
Shujinko should really be a personalized character (I'll say it again, if THQ can do a create-a-wrestler, Midway can sure as hell do a create-a-character!). At the start you would select his two fighting styles, from the collection of existing fighting styles in the game. During Konquest, you would assign skill points to not only these styles but special moves from a skill tree as well. The first skill points would be slower moves that would do lesser damage, and all would improve as you assigned skill points to your fighting style skills and special move skills. There is, of course, the danger that this is badly done, and you'll end up with a overrated monster that can take out everything in less than 5 seconds. This is where Midway has to be smart. Experience levels must be easy to gain until a certain stage as been achieved, and then they must be really challenging to progress further. When you finish Konquest you must still have a slightly weaker character than the ones already in the game, only through battling further in Konquest and finding hidden quests you would be able to make your character really powerful. Go play Diablo for a few hours, and tell me you wouldn't love to customize a character in MK like you do it in Diablo.
Konquest should also change a lot. I don't like the training missions, they're boring and mashing buttons isn't really challenging. Change that into it's own Kombat Training Mode, and make the central point of Konquest actual Kombat with various characters. Have different sorts of matches, without the handicaps that MK: Deception's Konquest has. I'd rather have Endurance matches than matches where I only deal 10% damage and the other guy is healing and does 400%. Of course, for the harder to get rewards, that would be an acceptable scenario. Unlocking a character, in my opinion, should be easier than this scenario. Also, Konquest should really be an adventure, and not just "go there, go here, do that, do this". Get rid of the big green marker, it just gives away what you really have to do too fast.
Konquest should mainly be used to create your profile's custom character and to get the Krypt Keys (get rid of the damn koins, all they do is slow up the Krypt with the annoying but necessary to avoid cheating auto-save features). Training should have its own separate mode, without the silly "Scorpion says: GET OVER HERE. Will you get over there?" lines. Maybe you could also unlock the training sessions in the Krypt, just don't make it part of Konquest, it's too tedious.

* Puzzle and Chess Kombat *

If you could keep these modes without having to take out things from Arcade from lack of time, DON'T INCLUDE THEM. I'd rather have 2 more characters in Arcade mode than 2 more game modes that I won't play anyway. Sorry Puzzle/Chess Kombat fans. These games are fun, but they shouldn't be made a priority to other more important features: the fighting engine, the characters, the story, the finishers (this is, after all, Mortal Kombat), a proper Konquest mode.

* REAL SECRETS *

Unlockable characters are fun, it gives you a sense of purpose to the whole Krypt and Konquest mode, but there should be secret characters as well, not in the Select Screen, and HARD to get, without getting excessively hard (excessively hard would be "Be in Earthrealm C-9 on top of the mountain between 9:00 AM and 9:02 AM during a Thursday that is also the seventh day of the month". We don't need this crap, we need challenging stuff, not tedious or impossible stuff. Puzzle solving, hard as hell but possible fighting, etc. Sure, time locked events are also challenging, but there is the risk of falling into exaggeration.

I guess that's about all I can remember as of now...
Off to play Deception I go