Single sign on across a bunch of forums was always a holy grail. Users don't want to register for 10 forums either. I remember EZBoard (a free forum host in the early 2000s) had an EZBoard Gold membership where you paid monthly for SSO across all EZBoard forums instead of having to register on each forum.
Traditional ads (Adsense) are pretty much dead on forums except for breaking even + pocket change. And unless your forum attracts a lot of guests, you're stuck in the position of showing increasingly crappy ads to your own users, and of them the few users not running ad block. It's kinda cruel.