Uhhh, no? I don't see how it adds more appeal. People see an 8 sided cage and think 'oh shit'. The UFC is known for it an its a part of its brand ingrained in their very essence. Try to picture a UFC event with out the Octagon in the middle you'll find yourself confused and befuddled.
Also, I don't see how a ring makes things 'more official', what not with guys getting tangled up in the ropes and thrown out of the ring. The idea of a cage is elegant and simple and eliminates the non-sense in that regard. Even in the land of MMA rings, Japan, Dream decided to try out the cage.
The UFC are pretty much sanctioned everywhere (barring some notable exceptions), I don't see why they would change their central symbol to appease the minority of people.