Hi-DPI/retina ads

I am curious if anything's in the works for high-resolution ads. Our site looks great on Retina Macs and hi-DPI phones and tablets but the ads look fuzzy since they are still at 72 ppi. Maybe have the ability to upload something proportionally larger (2-3x) in the same spots (like a 900x750 would be allowed in a 300x250 spot and smashed to fit)?