The donkey would be a neat addition.
But for a Long time I've hoped for the non-downforce slick-shod Caterham racers that really do compete with large grids accross the world.
Even if we got the Donkey, it just wouldn't fit with the GTR class. The car is So diminutive in size, it just needs a class of its own. Also The tire wear would just not be comparable to the others, 900 lbs vs 2000+ lbs.
To me the only thing holding back the Micro-GTR class is that they're FWD. You bring any FR or MR, even 4WD into the slick-without downforce and I think it would really become a popular class.
It could be a natural progression for something like the STCC, low/no downforce ~touring level cars.