I actually prefer this, because other special Pokemon forms like Ash Greninja and Notch Ear Pichu are stuck forever in the games the events are for. This form is should be able to transfer to the generations after Sun and Moon.

I expect this won't be the only new form for existing Pokemon in Ultra S/M.

I don't want the mystery of what's under Mimichu (or however you spell it's name) costume solved, but I could see it appearing.

Probably more Alola forms too!