I do not think EY would join any alliance.
All the 3 big middle-eastern airlines - EY, EK and QR focus on transit traffic, competing with European and Asian carriers. Thus I think they add very little value to an alliance. There are very few destinations that are covered by these 3 airlines and not covered by any of the 3 major alliances.
If at all they do join, considering the relationship they have with AA, MH, I'd say oneworld is a good bet.