I mean, Wade's the one calling the shots on this, but I've always said that I feel like Retro is fundamentally different than a lot of other potential subforum candidates since:
it's inherently something that you don't expect frequent, sustained activity on (meaning that the threads will sink)
it's not a topic where there's going to be an exogenous cue for everyone who cares to go find the relevant thread (the way that a WWE PPV event will make the wrestling fans go see if there's a thread, for example)
and that you therefore need it separate to keep it visible.
As for Discord, Discord moves too fast to be of use for Retro (see the first bullet).