Not for not, but Parapet is normally costed as , even as a common. This is a Parapet with a drawback (if you kill the enchanted creature, you kill the enchantment, too.)
It's a weird problem. I can get why you didn't go +0/+2, because the card stops being a common at that rate (maybe?). I'm not sure how you solve that, and keep the Ambient relevant. I mean, if the Ambient cost , then why would anyone bother paying the for the non-ambient version?
On a separate side note, do you have anyway of dealing with the memory issues this card creates? I know what I'd do: Ambient would lay ontop of creatures, while non-Ambient would lay underneath. It's not exactly something you can enforce, though...
I'm afraid there's no way Glorious Anthem works at common. I've playtested custom sets where people put Glorious Anthem effects at common, and those cards just really skew things. This is a Glorious Anthem that ramps up to be Collective Blessing or larger? This would be perfectly suited to rare.
Note that Overload from Return to Ravnica gives us some examples like Mizzium Mortars and Cyclonic Rift where one mode of the spell is a common effect, the other is rare, and so the overall spell has to be rare. The same will apply here.
You may be interested in this discussion over on Goblin Artisans about the Illustrate mechanic, which is very similar to your Ambient mechanic. There was a lot of disagreement in the comments about whether it's possible at all to make commons with Illustrate, whether they have to be ridiculously low power level, and so on. That thread will give you lots of food for thought as you work through your designs here :)
You already mentioned that Splendor is a functional reprint of Heroic. However this card is a lot better than Wingsteed Rider - and has a lot more gameplay complexity as well. I suspect it's likely to be best if your common Splendor creatures only affect themselves.
My first thought was that this isn't really suitable for common. But then I remembered Totem-Guide Hartebeest is common, so... okay, assuming chill factor is as common in this set as devotion was in Theros, this probably does actually work at common.
you can do the opposite keyword that turns it into an Aura. so it's only an aura only if you paid the <keyword> cost.
Not for not, but Parapet is normally costed as
, even as a common. This is a Parapet with a drawback (if you kill the enchanted creature, you kill the enchantment, too.)
It's a weird problem. I can get why you didn't go +0/+2, because the card stops being a common at that rate (maybe?). I'm not sure how you solve that, and keep the Ambient relevant. I mean, if the Ambient cost
, then why would anyone bother paying the 
for the non-ambient version?
On a separate side note, do you have anyway of dealing with the memory issues this card creates? I know what I'd do: Ambient would lay ontop of creatures, while non-Ambient would lay underneath. It's not exactly something you can enforce, though...
I'm afraid there's no way Glorious Anthem works at common. I've playtested custom sets where people put Glorious Anthem effects at common, and those cards just really skew things. This is a Glorious Anthem that ramps up to be Collective Blessing or larger? This would be perfectly suited to rare.
Note that Overload from Return to Ravnica gives us some examples like Mizzium Mortars and Cyclonic Rift where one mode of the spell is a common effect, the other is rare, and so the overall spell has to be rare. The same will apply here.
You may be interested in this discussion over on Goblin Artisans about the Illustrate mechanic, which is very similar to your Ambient mechanic. There was a lot of disagreement in the comments about whether it's possible at all to make commons with Illustrate, whether they have to be ridiculously low power level, and so on. That thread will give you lots of food for thought as you work through your designs here :)
You already mentioned that Splendor is a functional reprint of Heroic. However this card is a lot better than Wingsteed Rider - and has a lot more gameplay complexity as well. I suspect it's likely to be best if your common Splendor creatures only affect themselves.
My first thought was that this isn't really suitable for common. But then I remembered Totem-Guide Hartebeest is common, so... okay, assuming chill factor is as common in this set as devotion was in Theros, this probably does actually work at common.