While I'd very much like a 3rd one, I figure maybe they'll be tempted to try something else, the older divinity games were all fairly different so they might not want to make just another crpg with turn based combat. If they do plan one though I'll buy it. I'm also a bit surprised they kinda didn't have plans for some DLC content(or at least, they haven't said anything yet).
As far as I understand, Larian head Swen Vincke pretty much said that project E (which later become Original Sin) was something they wanted to work on all along. Even the first Divinity was meant to have something like this, but they had to make it closer to Diablo to sell it to publishers (and that's also the reason for the terrible "Divine Divinity" name)
The Original Sin games are also funded partly by fans, so totally independent from publisher meddling. I could see them continue making these kinds of games forever. Especially as they are changing the formula quite a bit. DOS2 introduced new elements to the combat, much better companions, actually good writing and 4 player co-op that allows each player to work against each other and form short lived alliances and back stab each other again.
So even if a DOS3 comes (I'm fairly certain of it), it will certainly bring many new things on the table.
