How about having fights in a ring but instead of ropes, you have a chain link fence all around? The cage will be as high as where the top rope would be.
This will be a single integrated structure as opposed to some WWE setup with a steel cage surrounding a roped ring.
You have the benefits of the cage without fighters falling outside and made to restart in the middle.
a square cage will mean more guys get suck in the corner as there would be no flexibility to lean back. It would favour wall and stall too much and create more of it.
The height has to also be enough that if leaned back on or pushed into you don't tumble over it.
How about a picket fence? 
how about a windowed octagon... like hockey. bye fence grabbing
Lmfao at Stu straight pistol whipping Orcus on this thread
thunderdome...except made out of plexiglass
Only problem with plexiglass would be that it kills the fighters the first time they get blast doubled into the wall.
I like the plexiglass idea- I believe it could be reinforced to be completely solid. It would make viewing the fights an incredibly uniquely different experience, that's for sure.
However, a reinforced steel & plexiglass frame around The Octagon brings up concerns for fighter safety, in my opinion. You could easily see a fighter knock a guy out by finding a way to inadvertently slam the back of their opponent's head into reinforced plexiglass... and shutting them down from there, after most likely severely concussing them from a slamming blow to the back of the head. I definitely would not want to see that happen.
On second thought... I don't think the plexiglass would work. But it certainly has potential. A VU for your efforts, sir.
Plexiglass also would have smeared blood and sweat constantly, plus Windex and a squeegee would need to be worked in by some ringside teenagers. I don't like that.
Go with the vale tudo ring with the netting on the lower portion. Stops wall-and-stall and adds better visibility, but also prevents restarts. 
ChaosOverkill -I think it would cancel enough jab n jog to make a little more wall n stall worth it.a square cage will mean more guys get suck in the corner as there would be no flexibility to lean back. It would favour wall and stall too much and create more of it.
The height has to also be enough that if leaned back on or pushed into you don't tumble over it.
Why?
A cage as it is is perfect for MMA. No point in changing it.
MMA should never be held in a ring.
FadeToBlack - Plexiglass also would have smeared blood and sweat constantly, plus Windex and a squeegee would need to be worked in by some ringside teenagers. I don't like that.The octagon girls could have sponges and spray bottles with round numbers on them.
Go with the vale tudo ring with the netting on the lower portion. Stops wall-and-stall and adds better visibility, but also prevents restarts.