It should work like in X and Y:
Increase attack power
Make it permanent until Lucario if K.O.'d
Still takes damage
Basically, this is what I've been thinking. Mega Evolution is supposed to be this one big push to get an advantage in battle, and a Pokemon is unable to revert back until 1. the battle ends or 2. they faint. It should also be noted that a Pokemon that has Mega Evolved, faints, and is revived within the same battle is unable Mega Evolve again. Thus, I honestly don't think it would fit as a Final Smash, something that can be repeated multiple times in one battle. I like the idea of only being able to activate it after you take a certain amount of percent, though, which has been brought up. However, like in X and Y, it could only be used once per match. I think a good benchmark would be 100% personally, but maybe more? Also, the Mega Evolution doesn't have to be THAT big of a boost, just enough to make a noticeable difference without being too crazy.
Anyway, this would definitely make Lucario a much more interesting character imo. Do you try to get an early lead, or do you wait until your last stock to make a comeback? If Mewtwo makes it in (which I'm pretty positive it will), he could also utilize his Mega Evo(s) in the same manner. Of course, that's just what I think would be cool.