It'd be a huge mansion.
As you wander in there's a long corridor filled with doors. The very first door you come to is the GT5 section. Toddlers are running about throwing food and cuddly toys at each other, while some are taking photographs with wind-up cameras (photo section), some are having sprint races (drag section) and some are walking around sideways (drift section).
Then there's a little ladder leading to the attic, where the previous and forgotten GT games reside.
After that is the other sections. iRacers are in groups laughing about GT5, while the occasional fanboy who snuck in is desperately defending their precious PS3. A large room filled with all different people, all sat on different groups of chairs in groups, discussing their favourite games.
Next there's a staircase leading downstairs, to the Automotive section. One area is filled with concepts and the newest cars, which is the news section. Next is an area filld with random cars, with members showing off their rides, and the occasional person 'My Bugatti's in the garage right now, but I own one, honest!', which are the Cars in General/Members Rides sections.. Finally there's a group of race cars and a little Go-Kart track, plus maps and pictures of famous circuits, filled with people discussing Motorsports.
Finally there's the Off-Topic section. The Rumble Strip is a lounge, with people sat sipping drinks and discussing their day, their hobbies and telling stories. The Opinions and Current Events is a bar, filled with people having intelligent discussions and drinking fine wines. There is then a room filled with computers, TVs, iPods, iPads, and people building computers or inquiring about problems they are having with technology. The Music section is a room filled with people lounging about listening to music, and a small recording studio. The Films and TV section is a home cinema, filled with DVDs.
At the end is a door guarded by a bouncer. If you are granted premium you are escorted to a pool where there are people ordering free drinks, girls dance in bikinis and there are a few drunks in the corner rambling about things, while other stuff non-premos could only dream of goes on.
Finally, is a seemingly innocent bookcase which is actually a door. But it is a door so secure only a select few can open it. Behind the door lies secret technology, like the BanHammer!(tm) and infinite supplies of padlocks...