If it's a factory revolver, I've been fortunate enough to be able to buy what I've wanted. There are a few more I'd have if they were a better value - meaning I want them a little, but they cost a little more than a little, so I don't have them.
Overall, the only ones I really want which I haven't bought because of price are all custom revolvers. My current "desire" is a swap cylinder GP100 done by Reeder, with 256mag and 255 Banshee cylinders.
I suppose one project I'd like to finish, which I just haven't yet, is transplanting the 5 shot 480Ruger guts out of a Super Blackhawk into an old Large Frame Vaquero, to have an "all Ruger" 480R Vaquero. I have the 480 SBH, and I have old Vaqueros which could be converted, but I haven't committed the time to doing the transplant quite yet. Only real reason is I'll end up with an SBH frame with minimal resale value, so the net cost to me is higher than I want it to be, right now.