First off, while she does have a big fan base within the Mario community, that doesn't necessarily mean that she has a big fan base within the Smash community, which is where Sakurai is going to be looking for newcomers in.
While Rosalina does pass the second, third, and fourth criteria, she fails the first (and the hardest) one as the requests for her pales in comparison to Toad, Bowser Jr., Paper Mario, and Waluigi, not to mention that she has less potentials for appeal than any of these four. Sakurai will also upheld a very high expectation for Mario newcomers and Rosalina does not meet that. The only two games where she is important is in the Super Mario Galaxy series and now that looks to be over, it is likely that she will never again appear in a mainstream game.
I seriously doubt that Rosalina is going to be in SSB4. If there is a Mario newcomer, Sakurai will likely pick either Toad, Bowser Jr., or Paper Mario for the spot. Even Waluigi is more likely, but given how little he has accomplished, his chances are slim.