I take it your going on the Dave Yates course… The one currently with the 12 months+ waiting list?
I know he asks customers for his own frames to bring their current bike so they can look at the setup/Geometry and decide if there’s room for improvement, why not build the ultimate replacement for whatever your best “workhorse” bike is, see if you can improve on current perfection… If you don’t “need” a new bike but have to build a frame maybe consider replacing one of your existing fleet, given the cost of the course it seems a shame to waste time and money building another niche machine, which bike do you use the most? Is there any imaginable way it could be even better?
Personally I’d love to have a crack at a full 853 version of a late 90’s, 18”, kona/voodoo frame but with more modern adornments, Disc only, ISCG tabs, full hose tabs (to run continuous outers), maybe some sort of proper mounting for the battery bag of DX/similar lights near the head tube, crud catcher bosses in the down tube, Basically a bike I know I’d use more than anything else for years with nifty little features I'd find handy…
Failing that make something so achingly beautiful it would be a sin to ride it, I’m thinking a BMX with Chrome finish Fleur de lis lugs and the richest possible blood red paint finish for the tubes, possibly with gold leaf flecks, internal cable routing and hidden a seat clamp… Mmmmm