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.)
Eh. I think they would've held off on Roy and/or Ryu if they didn't think they could put a new character out in August. Would be kinda dumb to release a bunch of characters at once and then go cold for an overly long time before releasing anymore. And I don't think Wolf, Ivysaur, Squirtle, Snake or Pichu would take considerably high amounts of effort. They could've made them by now.
Also, I think Snake's chances aren't horrible. I wouldn't say they're good, but I'd say their better than most newcomers and arguably some veterans.