I suppose with the super form can of worms somewhat open, we should consider which, if any, Megas should be implemented. Diancie feels like it's near the top of the list since as a gen 6 mon the base form may not have been designed to be fully independent. The other main groups I would consider are:
:lucario-mega::rayquaza-mega:: Significant parts of their games' lore.
:salamence-mega::gallade-mega::gardevoir-mega:: Referenced by Paradox mons
:sceptile-mega::blaziken-mega::swampert-mega:: While the XY starters don't have the ability to show off the gen 6 mechanic, the ORAS starters do.
:salamence-mega::gallade-mega::gardevoir-mega::metagross-mega:, arguably :absol-mega::gyarados-mega::camerupt-mega::sharpedo-mega:: Used by major NPCs in their debut games.