It's a common consensus that all 18 starter Pokemon will get mega evos. However, from what they've shown so far, no starter mega evos have been released except for the Kanto starters and Blaziken, and the Kanto starters will be given in-game, and Torchic will be in a giveaway. Nothing has even been said about the other starters yet. So I think you have to be pretty optimistic at this point to think the other starters will be receiving the same treatment.