I'm thinking with the rise of Mega forms will also bring up the rise of things like Prankster Trick (Thief being another option, but Trick being superior as it gets around Sucker Punch.) to accommodate shortly after. The way I look at this is pretty simple. Several Mega Forms will likely gain their own movesets. What's the most viable way to remove Mega Items? Prankster Trick (or fast Pokemon that learn trick in general). Then Pokemon like Mawile would become essentially competitively useless in OU. It's like Eviolite Chansey, but with Pokemon that are much more likely to sweep an entire team during the switch between generations.
If
this link is correct, giving MEvos a Choice item via Trick/Switcheroo could get shaky depending on the MEvo you are dealing with since there'd by no way to de-Mega evolve. I might Trick an Aerodactyl-M since that hasn't been seen since 3rd gen. but I'd be more cautious to Trick a Lucario-M, who can have specialized coverage and even if it doesn't, a Scarfed/Banded Adaptability STAB Close Combat coming off possible 140+ Attack is a very scary thought. Resistances don't really matter at that point.
All that being said, I'm starting to have second thoughts on the viability of some of these competitive strats people are talking about. I don't mean to put a damper on the thread, but strats/combos I've seen mentioned on Smogon like Hydreigon + Doublade is lol if everything that was banned from OU in BW1/2 gets dropped to OU when Smogon does it's initial tiering of XY. Traditionally, it has happened this way to give previous banned Pokémon a chance to see if they are still ban worthy or not (I guess). I just see more of the same familiar metagame we had in early BW where Excadrill and Thundurus-I run the show. The new game mechanics we've seen so far help defensive strategies with the exception of Steel losing it's Ghost/Dark resistance, which is pretty major I admit. But, unless we get actual Pokémon that can stand up to the old threats (Excadrill, Thundurus-I, Landorus-I, Tornadus-T, etc.), it'll be more of the same heavily offensive meta we've had in the current gen...
...except that DragMag will kinda suck lol.
I could be totally wrong on this though. I really hope I am.
EDIT: Totally didn't think about the possibility of Tricking the Choice item as the 'mon with the Mega Evo comes in or using Trick on that turn the player decides to Mega Evo. In the second situation, it comes down to priority: does the Mega Evo happen before everything or not? My point still stands on AFTER the fact though.