My picks are mainly based off characters I wanted to see but could settle to see as skins for Superman. I will also explain why I think the skin could work and why I want it. (Note: These are not the only skins I want just the main one's I'd like to see for Superman, I would also like to know yours)
1st: Mon-El I feel that Mon-El is a simple color scheme edit and legion is cancelled so this would be a nice payback to the character
2nd: Superboy, Superboy just like Mon would be a simple edit maybe alittle less simple but the character is well liked and I'd give him his Black tshirt design.
3rd: Eradicator, I like this character alot and feel that the cape tucked into the S in injustice was similar but I'd rather see the character skin, selected appearance you could decide upon.
4th: Superman Beyond, I really did not think about this one but Batman Beyond skin is in the game so why not.
5th: Powergirl, it might seem silly that someone would suggest power girl for superman skin but if you seen the trigon skin for raven you'd know it's possible
6th: Steel, Yes he has a mallet but that doesn't need to be shown and his suit was made to mimic Superman's abilities. Design you could pick.
7th: Superman 1 Mil or Superman 3000, a golden superman would be epic or 3000's design for advertising the Justice league 3000
8th: Lor-Zod (Nightwing), I'm not that strong for this one but he's my favorite superman character so I'd support it.
9th: Entropy Aegis Armour, I know I mentioned Steel but this suit would be awesome maybe not shown as powerful as it is but awesome to use
10th: Supergirl (Kara-Zor-El) she's trending right now due to the prequel comic for man of steel, she has her own title She should actually be a DLC.
For the Steel and Power Girl I'd say that would be what needs the voice changes the most.