I have a love-hate relationship with the existing MP-E65 so would love to see something like that without its current operating characteristics. Biggest ones I'd like are: make it parfocal and can I have more working distance please? Well, any working distance at all. I have hit subjects when trying at higher magnification before... I'd say 100mm focal length minimum, preferably 150mm or more. I would be happy to lose a bit of maximum magnification if it helps, as I rarely find going above 3x to really give much more out.
At the other end of the spectrum, a lower magnification macro zoom would be nice. Imagine a 70-200 f/4 macro! I don't mind losing AF in the macro range, while still allowing the lens to be used as a normal telephoto.