Show the roster immediately - in chunks I guess, so starter characters first, followed by 5 characters every other week - so people can stop posting BS leaks every now and again. After that, people can whine and moan all they want 'cause there's nothing I will do to change the fact it's finalized. Hell, I might even pick out characters people want and tell them why they're not in. I will be a mean developer. Heh heh... MUHAHAHAHAHAHAHAHAHA!!!
Oh, you wanted Cloud Strife from Final Fantasy VII? Are you that stupid?
So you wanted Proto Man? Go make your own game.
Tingle? No.
That being said, that doesn't mean I won't listen and include certain characters that people rationally justified or that I was not aware of that existed. I will say why certain characters aren't in if technical issues were involved. So, let's say the roster draft called for Four Swords Links - four characters fighting as one -, but it wasn't possible on the current tech. I'd mention that after the game's release or if someone coincidentally asked for them.
Besides move set changes, especially if they're called for e.g. Bowser and semi-clones will be prioritized, I'd make it so costumes exist, but only if they relate to the character. So, stuff like palette swaps of Samus's suits are in, but if time allows it, I'd make her Light, Dark, Fusion, and Dark Suits look the part - Dark Suit would have to be based on the PED Suit since it'd be confusing to have a Dark Samus Assist Trophy and Samus running around at the same time in chaotic battle.
Character appearance would just be based of their latest game - main series over spin-offs unless there's a major gap between releases. So, we'd have a Skyward Sword Link and Zelda, but Sheik and Ganondorf would be based on their Twilight Princess looks, however, they might get costumes from their old games, so Twilight Princess Link and Zelda will be there, but the default is SS Link and Zelda.
Oh, and let's say in the future, there's a ton of characters. At that point, I'd make a franchise selection screen or something to lessen the insanity of 50+ characters. It'd be customizable so if you want an overload, go ahead, but if you want to streamline them, make it so it's only text, etc., sure.
Also, Lucina with her own move set immediately - or give Marth a fencing move set, while Lucina takes the old moves; still would change the specials between them. Link and Toon Link would also get different items from recent games so they don't have the same specials. Some things would stay like the bow and bombs, but I could make it so they're not specials - down smash could have Link roll a bomb or drop it 8-bit style. Basically making characters a bit more unique. Hmm, maybe Luigi should get the Thunderhand thing from Superstar Saga...
My first and arguably only priority would be to scrap the 3DS version.
Or just make it so the 3DS and Wii U have different characters, stages, and more so they're not just a handheld/console version of each other.
For example, the 3DS would have Vaati, but the Wii U would have Ghirahim for newcomers. The 3DS might be more singleplayer orientated, but that doesn't mean there's no multiplayer - Smash Run seems to be that idea. The Wii U will be more multiplayer orientated and a tournament system would be intricate, but easy to use along with mulitple ways to play multiplayer - local, online, tournament, ranked, fun, grouping, etc.