Aery and Comet are basically the same, Aery can't be dodged though. All they do is "more damage" (and Aery can shield wooo)
If you take a look at other trees, they all have different keystones doing different things (but won't deny most of them do more damage)
Won't list it, since it's obvious enough.
Here's my keystone proposal :
*random magical name*
After damaging a champion with a spell, the damage dealt to the enemy champion by your champion increases by 2% per second, reaching a maximum of 10%. Once 10% are being reached, a comet will fall down around the enemy (not dodgeable) and will damage units all around him
I think it would help DoT mages by a lot, which atm don't have any reliant keystone (aside of Aery obviously)