@Trainer Frankie: Only new moves, abilities, and items, and Cosplay Pikachu, can't be traded to XY over PSS. Cosplay Pikachu is also impossible to send to Bank. Pokémon with updated movesets (provided the move exists in XY) CAN be transferred over Bank and traded thru PSS.
@SapphSabre777: It's confirmed that any Rayquaza can be taught Dragon Ascent, not just the one you're required to catch in Delta Epsiode. Zinnia's grandmother, who hangs out in Meteor Falls post-Delta, will teach any Rayquaza you bring to her Dragon Ascent, allowing it to Mega Evolve.
@Momopeach: Post-game, there's a stone salesman on Rte. 114 who will sell you the other starters' Mega Stones. They're cheap, too; only 1500 Pokébucks. Altho he sells Hard Stones for 40,000 Pokébucks, too, and his items aren't clearly marked, so be careful what you click on.