I think not and I hope to all that is Game Freak that it will not. Fusion works for most other franchises mostly because they don't usually have such a large fanbase compared to Pokemon's, allowing them to get away with it and also if it's a staple of the franchise then it's usually accepted.
Mega Evolution was a really interesting, new and most importantly acceptable feature because it was pretty much temporary and doesn't interfere with the main evolutions. Fusions would have to be the same and not a permenant feature IMO.