While I'm not a huge fan of the .204 either, I do own a couple.
I guess if I were building one, I'd likely go for a wildcat. I suppose by now, every reasonable case has been wildcatted to .20 Cal. So, given that thought; I think my pick would be either the .218 Bee necked to .20, or the .221 Fireball necked to .20...
Just because...
GH