I'm not sure that's really possible; the Mac Pro is a specialist three processor machine, using twin GPUs for massive compute/graphical power plus a high end CPU to make it all work (or for the computation that the GPUs can't do). I'm not sure there's much danger of the Mac Mini creeping in on the Mac Pro's territory as I very much doubt we'll see a Mac Mini with more than four cores any time soon! There are also other factors such as the Mac Pro's ECC support, more I/O etc. that makes it the only option if you really need a workstation/professional type computer.
At the same time I agree that the Mac Mini and iMac aren't directly opposed either; the Mac Mini is still very much about people that already have a monitor, or don't mind having a separate monitor, while the iMac is an all-in-one. I fully expect over time that the iMac will lose its discrete GPU, as integrated GPUs are just getting too competitive, and the iMac has never really been a gaming machine (even if Apple sometimes mentions its gaming performance).
I'd like to see the Mac Mini and iMac be made directly comparable; i.e - the Mac Mini would simply become a headless iMac, ideally updated at the same time as well, and perhaps missing the high-end (with discrete graphics) option, for as long as the iMac retains that model. It would make a lot of sense for Apple to do a MacBook Air level Mac Mini for tempting budget switchers and just generally anyone looking for a modest desktop, as the savings would be a bigger portion of a Mac Mini's price, compared to an iMac which ends up only slightly (proportionately) cheaper for a significant drop in performance.