Futo could also get into class D as their second unique car and create a new unique for class E. But I don't know if it would be possible to transfer all the top times with Futo into class D, probably not.
Another solution would be to buff the other classes until the gap is big enough, but class B is already too close to A imo. And buffing literally every car because of Futo seems like way too much work.
Maybe wiping the Futo times wouldn't be such a bad idea. Good drivers that made the top 5 with Futo will be able to get into the top 5 again, with another class E car or a nerfed Futo.
Futo is truly close to class B in some races. I recognized that yesterday when I made the top of all time for class B in Street-X Docks 2 [R], I made 16:18 with Super GT, and Futo's record got 16:93 on the same track.