The new feature sounds cool and all but I think the only way I'd buy this on the first day if we get new alola forms that I think are ultra cute and/or megas/z-moves for my favourite pokemon (Wigglytuff/Jynx/Gothitelle) or if there's going to be something major with regards to Lillie.
A concern I'm having with USUM is that they'll use the alternate timeline excuse to retcon the story. I quite liked the story and wouldn't want it to be changed too much. As such, I'm hoping they further the story in this alternate timeline as opposed to changing the story completely.
My first thought is that this is so unnecessary but if they take the opportunity to expand on SM's story and give me Alolan Jynx alongside that vacation cruise ship where I can battle every Gym leader/Elite 4 member in the past again then I'll gladly buy this.