At this point, I don't really think a more powerful processor that handles more simultaneous effects or a bigger grid is necessary or even really desired by most Axe-Fx users. The Axe-Fx doesn't really need more processing power, specifically. It needs to be able to hold more settings within individual blocks.
When compared to the Axe-Fx, the *single* theoretical advantage of any other rig you can build is that within individual effects, you can have as many sounds "per patch" as you want, provided you get the right gear. Buy a Strymon BigSky and you have 200 reverbs available without having to change any of your other effects or amp channels, etc. In the Axe-Fx, you're limited to 2 instances per block.
So yeah, the best improvement for the Axe-Fx I can currently think of would be more available instances within effects blocks. Imagine being able to have 5 or 6 instances of the "Amp 1" block (instead of X/Y, having V/W/X/Y/Z,etc) in one patch. You'd be able to have a different 5 or 6 channel amp in every patch, along with 5 or 6 drives, delays, reverbs, etc.
Or maybe this could be done by expanding the Global Block's available memory and functionality to store 20 or 30 instances of each block, and then allowing any global block setting to be recalled and override its block in any patch. So for example, on any patch with an "Amp 1" block, you could press a switch on your MIDI board to "apply Amp 1 Global Block - 25" or whatever. At that point, you'd have 20-30 "favorite" amps available in every patch.