I used to lurk on the Pathologic subreddit, the overwhelming consensus there was people recommending they do exactly this and that Pathologic 2 already has enough content to be considered a complete game. I may peek in to see what they think now that it's actually happened though.

EDIT: apparently IPL has announced on Twitter that anyone who backed the P2 Kickstarter is getting P3 for free. And clearly this is different enough to justify a whole new game rather than, say, a paid DLC (and without charging for the Bachelor route they flat out won't have enough money to make the Changeling route).