@MarcParis an open air cooler inside that case wouldn't be a great idea, the hot air from the cards would just get dumped into the case and wouldn't really have anywhere to go - they would probably be hotter than founders edition cards. However if you want to try, the EVGA GTX 1080 Ti SC2 would probably fit.
For me personally though, air cooling the GPU's doesn't really seem to have any excellent solution, I'd rather just water cool them ( especially since seeing the water blocks in the vertical position looks so damn amazing ). The reason air cooling the CPU may be a good idea is because now days you can find some pretty efficient CPU's that offer amazing performance like the i7-8700 or an undervolted i7-8700K. Tbh though I'll probably just end up water cooling the whole thing but I'm gonna do a little more research on air cooling the CPU. I also prefer to have the GPU's water cooled so that they can be overclocked, and overclocking the CPU isn't one of my top priorities.