What could Mew be given, that is currently in game, to bring it up to par with Mewtwo? Most likely in terms of abilities and moves as I don't see them changing it's stats or giving it a unique hold item (unless through an event).
Give Mew STAB for all type moves since it's the ancestor of all Pokemon.
That would be an interesting ability. An easy way to make Mew Uber would be to allow it to learn anything from a move relearner/reminder NPC.
I think the STAB for every type move alone would make it Uber. The problem with Mew is that it just doesn't hit hard enough. With STAB on all moves it would be like the next Genesect. Rock Polish sets would be very effective and also Choice Scarf sets because of STAB U-turn and STAB on Ice Beam, Fire Blast, Aura Sphere etc.