Ah, I didn't make myself clear enough.
I meant that Victini appears to be the only 5th generation Pokémon that could logically receive another forme, one we haven't heard about yet. Forme gaining is usually reserved for legendaries or quasi-legendaries (which Rotom arguably is, with his Pokédex placement, one-per-game availability and general mysteriousness). Non-trio-master legendaries also rarely get any love. I don't think Reshiram and Zekrom will be changed when Kyurem will, and the Kamis and Musketeers aren't going anywhere. Meloetta already has an alternate forme, and Genesect IS an alternate forme per defini- oh.
Genesect-O, anyone? Just throwing the possibility out here, for later. They won't reveal an alternate Genesect before revealing the first one.
My money's on Meloetta Step Forme or alternately a new Victini.