The successor to OS 3.1 will be 3.9
The other choices, AROS, MorphOS, OS 4, will only get fractured support from the community, and there will never be any unifying event, or breakout winner. Eventually they will all fade into obscurity, but the retro scene will remain alive.
That scene would probably use 3.1 forever, and there would be no successor, but I predict, that
someday when Amiga, Inc. is bankrupt, and no one else to watch over the IP, that 3.9 will became the favorite as the last release that runs on emulators like WinUAE. People won't pay for it, but they'll pirate it freely, if, well, there is no IP owner to stop them and it doesn't hurt anyone, people who wouldn't normally pirate something, won't see objection to it.
Thats what I consider the most likely scenario...still, I don't think its overwhelmingly likely, maybe 10% chance...I just don't see anything else as being more likely to happen. I certainly don't *want* that to happen.