Android and iOS have been the leaders of smartphone OS systems for so long that it's hard to picture anything possibly taking over from them. For a randomly and completely new OS to take over then that OS has to provide a service which the two previously stated OSs dont, and that's not likely at all.
Frankly I can't see this happening but if it did then then it would add some spice to the smartphone market. With blackberry and windows operating systems both completely being phased out of existence Android and iOS haven't really had any competition apart from themselves.
I think it's possible but not likely for something like this to happen because taking a page from Nokia and their Symbian OS which was the pinnacle of smartphone operating systems only a few years ago but is none existent now, I'd say that anything is possible in the smartphone market.
I wouldn't mind if this happened but seeing as both iOS and Android are constantly fine tuning their user experiences, it would be awful hard for another OS to take over.