The limit on compression driver size is mainly its being an edge-driven dome, right? Why don't they turn it around and put a conventional motor behind the dome? Wouldn't that allow for gigantic compression drivers? Of course you'd have to have some way of having the dome shape on front but a nice strong coupling to the VC on the back, but that shouldn't be too big of deal. Would there be no merit to a driver like that?
This is just off the top of my head, so I may have missed a couple things.
I believe there are three things that we battle when designing a compression driver. The first challenge is maximum output. Maximum output will generally be dictated by displacement, as long as you have sufficient power available and your voice coil doesn't overheat.
So the obvious solution would be to increase xmax, cone size, or both.
But that leads us to the *second* challenge. Which is that your upper frequency limit will be dictated by mass. (read this:
http://volvotreter.de/downloads/Edgar-Show-Horn.pdf)
So we see that challenge one and challenge two are at odds with each other. When you try to increase output by raising displacement, you end up limiting high frequencies. And this isn't just in one dimension, it's in two. Raising xmax raises mass because the suspension gets heavier. Raising the cone area by making the diaphragm larger ALSO raises mass. (This is why TAD uses beryllium in their diaphragms, you can raise the high frequency limit by lowering the mass.)
The third challenge we face when engineering a compression driver is getting the entire surface of the diaphragm 'in phase'. That's a REALLY complex topic. Here's a pic of a compression driver that was cut in half, showing how the phase plug equalizes the pathlengths:
Long story short:
We have three competing criteria, and they're at odds with each other. If you optimize for one criteria, it's often at the expense of another. And that's why horns work best when each driver covers a narrow bandwidth. It's not possible to have one driver cover a wide range in a horn.
Well, I guess you could, but the efficiency would be very very very low.
(Back loaded horns can have a wide bandwidth with a single driver, but that's because 80% of their bandwidth isn't horn loaded; only the lowest octave is.)