With update 20, two Time Attack cars have been added to the game. I think that Time Attack cars make a great addition to Forza Motorsport, but currently they are grouped in Forza Specials together with the MX-5 Cup and a Holden V8 Supercar.
I believe these Time Attack cars deserve their own division, especially if more are to be added in the future. This will also help with setting up private mutiplayer and freeplay events with only Time Attack cars.
Also be sure to vote for more time-attack cars!