(Just FYI these improvements I came up with aren't grievances with MKA, because I love it and think it's AWESOME for a first go-around, these are just ideas for MK8 )
For the color option, rather than having a matrix of colors, have a full gradiant rainbow like in MS-Paint. That way we can have literally any color we want, not just something close to it.
Be able to adjust the body shape properties. Make them fatter, bigger feet, smaller head, wider shoulders, longer fingers, shorter legs, etc.
Give a series of textures for their skin/clothes to chose from. I know they give us the ability to give them different types of skin sort of....but that also changes they're appearance.
In the same vein as assigning them various moves, you should also be able to assign them their own specific fatality. Kind of like the KAF system now, only with many more options as to what happens when you press certain things. That way EVERYONE will be surprised when fatality time comes, because there'd be so many different options the person could have chosen from. They'll have TONS more memory space on next gen so it shouldnt be such a huge problem to do this.
Rather than give generic voice systems for them to use "Small male 1,2,3...etc" let us choose between using those type of things, or just using a voice system that's already in use for a real character. For example, when I was making Bruce Lee, the voices they gave me didnt suit him, but Liu Kang's would have.
One grievance I had with MKA that they DEFINATELY need to fix is give us the ability to use EVERYONES weapons. Unless I'm just a moron and am TOTALLY blind (which i can accept as an option

) I didnt see any nunchakus as an option for my KAK's weapon. I made bruce lee and gave him jeet kune do everything and was going to equip him with Liu Kang's nunchakus....but they're not there. Why not? They're already a part of the game engine. Just give me access! Anyway....
Other than that, just more of everything in every category. I'm sure whatever they do will be great though, it already has been IMO.