Well alternate costumes have been in mortal kombat since MK4, so they should definitely return. But only as extra skins, and not have different moves for each alternate like you described. What if the player likes the moves of the original costume but likes the alternate costume better? see what I mean... I would like to have at least 2 alternate costumes, plus the primary costume for the next mortal kombat. If DOA:U can have certain characters to have up to 14 alternate costumes, then having just 1 more costume per character should be a joke to midway. Right?

Well, in DOAU, the highest costume count is actually 20.

Costumes should return? When did they ever leave?

I like the idea of having more costumes like DOA, but, as Shaolin said, I don't like the idea of new moves for each costume.

Also:

"Also they should have a classic costume, like old MKs in 2D"

Do you mean having 2D characters in a 3D game? Because that would be a shitty and confusing idea. Having 2D characters in a 3D game/environment.

They should do what Tekken and Virtua Fighter did and give us the freedom to choose/buy what we want for our characters. Customize them so they always have a new and cool look. 2 Regular costumes and 2 Customized costumes would be nice.

Also they should do what Soul Calibur does and give everyone the freedom to pick what weapon they want to use for their character. Each weapon will have a weakness and a strong point.

That will be a Mortal Kombat game I will never stop playing.

New costume, New Alt. Costume, Classic Costume (from their first apperience).

It would be cool if the character had both of their costumes from the previous game, too, as a bonus to unlock.

I'd like to see them return but with more, Like Sonya looking like she did in MKSM (i think she looks cool just a bit more detail is needed) how she did in MK4! (best sonya look eva) and how she did in MKDA.. not that i was keen on that one!
Same goes for Mileena and Kitana i'd like to see them in a 3d version of the MK2 look! with there hair down and the UMK3 look as well as there deception and a new look!. i really want to see mileena with out a mask on!
Also the wepons should be attached to there costumes and they should not appear from thin air... i'm kinda hoping for too much aint i! but with all this memory they should have on these next gen systems they should be able to fit it all on!

They need to think morecarefully about which of the two costumes will be the primary one. in MKD, a lot of the alts should have been primary.

I would be pleased if each character had a trietary costume, but why stop there? You should have 4 or 5 outifts. If not that, then at least go the way of Virtua Fighter and Tekken, give us a decent arcade or tour mode to build up enough Koins and customize the characters.