I can see Sycamore's Garchomp mega-evolving. I can also see Ash at least directing it in battle at some point as the two seemed to develop some level of friendship. In fact I'd like to see Ash and Mega Garchomp battling Lysandre and his Mega Gyarados.
If Ash were to get a mega pokemon, then I think that Mega Charizard (X or Y) is the most likely, with Heracross following on close behind.
Regarding the odds of him catching further pokemon to mega evolve, I honestly can't say for certain. Everyone seems to think it will be Lucario, but I've had enough of that pokemon, especially after the whole Cameron fiasco at the Unova League. Personally I'd like to see Ash get a Gengar that can mega-evolve. It seems to fit him. Also Absol, though that may go to Serena... maybe.
Korrina will definitely have Mega Lucario though, and I think that Mega Gardevoir is guranteed to appear, but only if Diantha does.
Any others are up in the air, but I am fully expecting that a Meg Pokemon of some kind will be used as the main pokemon owned by the trainer who knocks Ash out of the league (he's gonna lose it one way or another, so he might as well lose to a Mega!)