Isle of Man shoot-out
.
A non-championship race (like the one that was planned for 20015-something?), with more than 32 cars allowed. A modern version of Tourist Trophy. But I fear that not many non-BTCC competitors would show up, as the cars seem difficult to unlock for a one-off appearance and the level of competition in BTCC is very high.
Otherwise a nice full grid of historic touring cars and drivers from the different eras at one (or a few) selected venue(s) would be nice. With good TV coverage, of course, and maybe racing for some kind of charity.