Does anyone know if the upcoming campaigns and whatnot (i.e. Gathering Storm) will be playble with only 1 or 2 PCs?...

I'm sure with a little work you can adapt it to do so.

I'd imagine with the core set supporting three players, making it fit 1 or 2 would be easier than previous versions which assumed 4-6 players.

With only one or two players, there is always the option of one or more of those players taking on two characters. This may not work if your players are novices in the hobby, but if they have some experience under their belts, then playing a second character can help round out your party, even if you don't have enough bodies in house to do so.

I imagine you could always supplement by running another party member as an NPC.