I think combineing several formus into a DM's forum would be an excellent idea, althjough i'd leave rogues gallery in bits and pieces. It gets a lot of use by the in character dm's and players. I think the d20 publishers forum should stay as is. Combining it with d20 games would make finding news from the publishers less stream lined. I think changing d20 games to d20/3e might work although most D&D stuff seems to go into general disscussion. A D&D: previous additions forum would be cool.
I don't think we need an OT forum at all, i think the non rpg sci-fi forum cleared most of that up. As to the {Hivemind} well, there's no need IMO to to do anything with it, let alone provide a specific forum. It's become pretty self moderating and obeys Piratecats rules, best bet is to just ignore it.

I'm not sure the Rules forum is all that nessary, although i never go there and tend to forget about it. Maybe that forum could get phased out and let general hand rules questions. You might get away with combineing marketplace and games seeking gamers into a personnal add forum or something like that. I'd also slap it into the non-rpg specific section. Then you could combine house rules,plots and places, and the coversion forums into the DM's forum and put that in the D&D/d20 system section. Finally after rearanging that stuff i'd change Bit's and peices to roleplay and have In character, story hour, fight club, and rogues gallery in there.
oh, and i'd pass on a d20 modern forum as well.
anyway, there's my two coppers worth.
