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How is this going to work? Here's what I think. Ditto will not have a mega form per se. Instead, it will only mega evolve once it has used Transforme on a pokemon which has a mega form. If it uses Transform on a pokemon that is already mega evolved, it will become the regular pokemon, having to use the mega evolution button afterwards. For example: Ditto is in battle with Mega Lucario, it uses Transform, it becomes Lucario and the Mega Evolution button appears in its menu.
On the other hand, I'd like to see a mega evolved ditto in itself, but that would need the addition of details, which would contradict the idea of the original design - that of undifferentiation.