Just posted this in the Pokemon Community thread, but I figured it would be good to discuss this here too. The way I see it, there are two ways this can be implemented.
Pokemon hold the 'Mega Stone' to access their more powerful formes for the duration of the battle. This is bad. Only certain Pokemon will have Megalutions, so this will more than likely cause balancing problems, as Pokemon with Megalutions will more than likely be much stronger than those without. Furthermore, if MegaBlaziken is more powerful than base Blaziken, suddenly Blaziken is obsolete. I really don't like the sound of that.
Now, option 2. Pokemon can access these formes if certain conditions are met in battle. This is a MUCH better option, especially if it can be somewhat skill based, because it avoids the base forms of Pokemon becoming obsolete - you start the battle with them and build towards activating 'Super Mode'. Unfortunately, there is no indicator that this is the way it's going to be handled.
Pokemon hold the 'Mega Stone' to access their more powerful formes for the duration of the battle. This is bad. Only certain Pokemon will have Megalutions, so this will more than likely cause balancing problems, as Pokemon with Megalutions will more than likely be much stronger than those without. Furthermore, if MegaBlaziken is more powerful than base Blaziken, suddenly Blaziken is obsolete. I really don't like the sound of that.
Now, option 2. Pokemon can access these formes if certain conditions are met in battle. This is a MUCH better option, especially if it can be somewhat skill based, because it avoids the base forms of Pokemon becoming obsolete - you start the battle with them and build towards activating 'Super Mode'. Unfortunately, there is no indicator that this is the way it's going to be handled.