I think the spiritual successor is actually Torchlight 2. It's made by the same people who worked on Diablo 2 and were not allowed to work on Diablo 3. The only real change from Diablo 2 is there's no real attempt to keep players on the same network and to ensure they're all playing by the same rules. So what you have is a plethora of Torchlight 2 games and many are following their own rules. People can cheat and create items at will. Beyond this, Torchlight 2 IS the spiritual successor to Diablo 2.
One thing you don't mention is Path of Exile doesn't use random maps. Torchlight 2 does. And I don't think Path of Exile has a single player game. Torchlight 2 does. Probably lots of other things. Oh, and Torchlight 2 can now be substantially modded because GUTS has been released. Can Path of Exile do that?
http://www.torchlight2game.com/news/2013/04/01/guts/
Torchlight 2 is freedom. No big man at the top to push you around.
Learn more here:
http://www.torchlight2game.com/