Aeon gun's increased range is massive - you can reliably kite enemy guncoms with it, and they're forced to either hope you slip up with the kiting (letting them get within range), have a load more nearby units to charge at you, or get enough T2 PD up to stop you. Oh and a cost decrease to gun upgrade is a buff to the gun upgrade.
Aurora I don't like but it does at least give a clear difference to the other factions who all have a very similar main tank.
Mercy - discussed a load in a separate threat - I'd prefer the oppsosite of your suggestion - instead of making them survive against MAA, make them die to MAA. IMO if the enemy has MAA by their com it should counter mercies. Making Mercies able to tank T1 MAA shots for a while would make them insane.
GC - It gets omni and the claws, which are really powerful against some matchups. As an extreme (unlikely) scenario - I can beat other land experimentals with certain SACU mass equivalent combinations. Try that against a GC and it'll easily crush them due to its claws [Edit: Forgot that the sandbox scenario I was recalling was UEF Shield SACUs with Percies underneath them and for some reason thought the claws got the SACUs - instead it got the shielded percies]. It also easily beats a monkeylord, and its 'crush when walking' is more meaningful due to its huge HP.
Czar - compare this to soulripper, there's just no contest. It's incredibly versatile - you can charge at the enemy com under shields with SAMs nearby and kill it (try that with a soulripper and you've just given them a big mass donation). You can kite enemy asfs with it; you can use it in a more attritional style thanks to its shield; you can do surprise attacks by storing air units inside it, etc.