Ubisoft has promised more games for the Vita and it makes sense having built an AC engine for it that another game would be forthcoming. This time we seem to have a distinct break between the officially announced Black Flag for consoles, with the mysterious Phoenix Rising seeming to be its own game.
Several leaks from Ubisoft like the image above and short-lived store listings, suggest this is for the Vita and looks to further pit the entities Abstergo and Erudito against each other. What era it'll be set in and who the hero(ine) will be remain very much up in the air.
Phoenixes have appeared in Egyptian, Greek and Oriental mythologies which doesn't narrow it down much. The chances of you getting to stab Dumbledore in the beard also seem pretty remote.