Had a lot of fun the one time I had the luck to have the arvo off to join, and have been meaning to thow in a couple of ideas.
First, the fun bits. With or without boost on, one of the nice things about this type of racing is that you are free to self-handicap in nifty ways. Might be as simple as completely turning the HUD off, using cockpit view, or actually using clutch and H-shifting. Also no-ABS (which could also be enforced room-wide for some races if everyone is Ok with it; friendly reminder to change BB is wise, though).
Second, cars. I do think doing one-makes is indeed wise, unless you have prepped a themed "vs" idea.
Just tossing out a few useful types (you've probably thought of many already and have them in store for later):
RX-7 Spirit. Maybe personal bias, but it's a damn fine balanced car. Premium S2000 is good also.
Old Muscle cars (eg Camaro Z28 - but you need really short tracks for these when using the stock gears)
Old anything chuckable, light, not too high powered (personal favourites would be things like the Merc 300SL, the Daddo Fairlady Z, Toyota 2000GT)
"Guest" cars (the always-available arcade free list) for surprise interludes (since no one needs to check/prep these at all)
Karts. Small open-cockpit types are fun, because you can definitely get the elbows out. These are particularly good when you want to use a track that normally sees minimal passing, because suddenly the track is effectively 2-3 times wider.
Oval racing. Not personally a huge fan of it as a spectator sport, but the 3-wide (or more) pack battling is challenging if the room is pretty full. Needn't be NASCAR; you'd want to turn the wick down a bit if so.
Lastly, if you do want to have any occasions without boost, one-makes are still pretty much the way to go, IMHO. The only reliable handicap I know of that I don't think can be gamed effectively is ballast by itself (it is also easily verifiable). Ya canna break the laws o'physics, cap'n. Limiter is OK as a fine tune if it's not masking better parts elsewhere.
Assuming you do want to have multiple cars available, allowing race cars/tuners/concepts vs road cars doesn't really work, in my opinion - best to have fairly similar car types on track. It is also my opinion that allowing higher powered or better handling cars on worse compound tires also doesn't work (forcing a worse compound on faster drivers in the same car sort of does, though).