I ran a Super Taikyu inspired championship on GT6, running a class that was basically ST5 with ST4 performance, called it ST200. It had only small FF hatches with 200hp / 900kg or 213hp / 960kg, on sport soft tyres. The racing was awesome, despite a wide range of driver skills, had some of my best races ever. Cars were : MiTo, Mini Cooper S, 500 Abarth, Vitz F, Swift Sport, Demio Sport, Fit RS, March 12SR (the first 2 being the ones with more power). All cars were within half a second per lap, tested on 3 different tracks.
Now on GTS it would be more difficult to build a ST200 class. Now you could build real-life Super Taikyu classes, but I'm afraid FF cars would really suffer regarding tyre wear, so allowing only them or getting rid of them would severely limit the ST5 and ST4 classes. You could always try to tune up the Roadster and the S-FR up to ST4 specs and forget about ST5 I guess. ST3 could be interesting, with the 4C and the new Alpine alongside the RX-7 and Supra, but the GTS roster will mainly fill ST2 and ST1 classes - and of course ST-X. Gr.4 cars can also fill the new ST-TCR and ST-Z classes.
But for all of this, you'll have to set up a lobby and control with your eyes and the kick button.