I picked up Payday as well. I messed around with the single player a bit and it's clear that it's meant to be played multiplayer. But I was checking the forums and some are saying that the online community is kinda shitty. Apparently if you don't have a rep of at least 100, you'll most likely be kicked off the server. That's not cool. I'll have to play some multi first to verify, I guess.
Cities: Skylines is exactly what I've been looking for!