This is an awesome idea. I've got a few thoughts about it that I'll just list in a slapdash order.

It seems like something that might cause undue server strain if anti-abuse mechanics aren't put in, such as limiting the number of "worlds" that can be created or some time limit on how often one can make a new one. I'm certainly no expert (or even a novice really) when it comes to dealing with servers and what they can handle and whatnot, but creating whole new side forums with their own sub-forums sounds like the kind of thing that asshats could abuse by making tons of them and crashing the site.

Mod level GM controls would be imperative for these things to work well, as would the ability to appoint co-GMs. On a second read I see you've already accounted for that, so all is well.

I saw that in all those specific examples there was some kind of registration system where prospective players would have to register a new account. I figure that wouldn't be the case here, but it gives me an idea. If they so choose, the creators of these side forum things should be able to bar posting from unapproved members in some sections or threads. Maybe this could fall under the general mod stuff, or maybe this could just be a wider scope use of the whitelist type feature you indicated that you'd like to add for RP threads, but there should absolutely be the option to bar random people from posting in the IC threads of these "world" things.

Alternative name ideas: Persistent Worlds (as that's kind of what they are from what I can see), Realms, Universes, Domains