Disclaimer: Bear in mind that the following is simply a rumor with no official statements from Niantic.
We all know Generation II is going to happen eventually. Several items are now lined up that support the Gen. II update going live the week of December 7, with the possibility of Dec. 7 being the actual launch date.
Silph Road has already confirmed datamining for Gen. II code that's currently in Pokémon GO, reported on Nov. 7 (coincidentally exactly one month before the rumored launch date):
https://thesilphroad.com/news/pokemon-go-gen-2-and-ditto-code-found
They have a solid track record in calling out updates (see: Ditto). And there's also the mining of a new sponsor (on top of ones mined at an earlier date), which lines up with with the next point.
Recently a crudely written internal memo from Starbucks surfaced with a promotional event included.
This coincides with a Twitter user asking for the Verizon promotion included in that same memo.
Both the Groupon and Facebook promotional events were confirmed and have expired already.
Lastly, with Niantic's campaigns with brand sponsorships (see McDonald's in Japan for instance), a partnership with Starbucks while heading into the holiday season is not farfetched at all.
Edit: Another thing to note is the recent rush to push out the "new" PokéStop-oriented tracking system. Starbucks PokéStops will be extremely obvious.
12/5/16: Some more potentially "leaked" info from Starbucks:
