I think it'll be one of three characters because I feel like Sakurai has used this "strategy" before. I don't think it'll be someone super popular but, instead, is someone who is more on the obscure side or has a small (but dedicated) fanbase. Why do I think this? Look at how Sakurai revealed newcomers for Brawl. Pit was revealed during the E3 trailer. Why? To build hype for a relatively obscure character. Fast forward 15 months; who was the newcomer that broke the newcomer drought after E3? Ike. Why? Again, to build hype for a relatively obscure character. Fast forward five months; who was the newcomer that ended the three month character drought? Olimar. I feel like this was deliberate especially when you consider how Sakurai chose to reveal significantly more popular characters like Sonic, Dedede, and Diddy between Ike and Olimar.
Fast forward to E3 last year. Who was the big shocker character at E3? Wii Fit Trainer. Again, I feel like Sakurai took advantage of the hype of E3 to build hype for a character virtually nobody cared about beforehand. Fast forward one last time to December of last year. Who was the character who broke the 6 month newcomer drought and the 40 day character drought? Rosalina, a character many people readily dismissed in favor of more popular Mario characters. See what I'm seeing?
I feel like Sakurai does this on purpose so the fans associate the thrill of seeing a new character after a long time with a character that isn't that popular or well known so that they'll actually be excited to play as them, even if they haven't a clue who they are or didn't particularly want them. As a marketing strategy, this is brilliant. In all honesty, would as many people have been excited for Wii Fit Trainer if she wasn't revealed at E3? Would Ike have received the fanfare that he got if Diddy had been the one who broke the newcomer drought with Brawl? This all being said, I expect the newcomer revealed to be someone who may not top many people's fan lists, but still be a well welcomed addition when they're recognized for breaking yet another character drought. If and when a Nintendo Direct is announced next week, I'll post my theories on who the three potential characters are.