Though I left this at CD for the sake of the cycle, it probably needs to cost at least one more. Generally, an extra color is worth about , I think, which means that Aven Fleetwing should cost , right?

Yeah, that equation broadly holds.

I note that hexproof is quite irritating on a flyer. Especially so if the monocolours aren't going to be able to block flyers at all, and will have to use spells or activated abilities to deal with them. This could end up being this set's Invisible Stalker, and I don't think we want that.

What if we nerfed the hexproof down to a "pay to target this" ability, somewhat like Frost Titan?

That's a good option. We should figure out whether the number of monocolour vanillas will leave us wanting some gold vanillas (well, flying vanillas, which is the most minimal french vanilla possible), and so whether to stick with Carer of Cloth and Water. But this is a more feasible possibility if it takes more mana to mess with. And perfect for the colours: taxing is typically a white ability, but Frost Titan is a powerful drag to make it blue.

I like the design of glimmerwing, and agree its great for UW but I feel we shouldn't pass up the opportunity to have a vanilla-ish creature where it makes sense, and I feel 2/3 is a very nice example; simply by being out of the range of 2 damage it makes it already a lot more resiliant. I'd say stick with Carer for now, but make sure a glimmerwing gets in at uncommon (probably with a different P/T).