I've heard august but I can't remember from where. And probably I mean, they shouldn't take that much work...
Let me know when they add the 3ds stages
You probably heard it in Sakurai's E3 "New content approaching!" presentation, same as I.

Man, I wish they would add some 3DS stages to the Wii U version... I doubt that'll ever happen, though. Sucks, because I would pay good money for Magicant, SNES Mute City, Balloon Fight, Pac-Maze, Rainbow Road, Prism Tower, and others...
Probably.
I'm guessing we'll get Wolf and maybe someone else in it as well. Plus Mii Fighter Costumes a plenty.
We'll almost certainly get a third wave of Mii Fighter costumes, but I'm not so sure about even more new characters so soon. Getting three new characters in one day at E3 was a huge deal, so I think it'll be awhile longer (later this year, fall or winter) before we see more characters added. I feel like Wolf will be announced and added to coincide with
Star Fox Zero's marketing during toward the end of the year.
After Wolf... who knows? Probably the character chosen from the ballot in Spring 2016 or something. Things will get very interesting and murky after Wolf's addition since he's the last remaining Brawl/Melee vet who is both highly requested and has realistic chances of being added into the Wii U/3DS roster. (Ice Climbers and Pokémon Trainer are out for obvious reasons, Young Link is just Toon Link but not nearly as cool, and only a very tiny minority even want or care about Pichu. Ivysaur and Squirtle could technically have a chance, but I seriously doubt it.)