They really need to look at League of Legends for the kinds of costumes that make players excited and interested in getting them.
If they made costumes the way riot does then we'd get tons of costumes for the popular characters and someone like Fang (or whoever ends up being the least played) would have one or two for the life of the game.
Also they can't do stuff like change attack effects like legendary skins in league or dota do due to the nature of the game compared to a moba.
If anything they do what they did before in sf4 which is just make sure everyone in the cast gets costumes so that anyone who plays the game gets something when new stuff rolls out.
Riots model works for a larger scale roster since they have to pick and choose a bit because there are over a hundred characters so they can afford to ignore some of the cast. Street fighter 5 even with a finished roster in a couple of years is still going to be much smaller than that and as we've seen with sf4 it's easier to give everyone some attention as a result.