While I don't think CO is anything special, it's neither underwhelming nor overwhelming at this juncture. I feel what you're suggesting for Diurnal CO is too potent and unbalanced but the Nocturnal adjustment sounds nice.

I'd be more concerned of any more buffs AST gets will throw out any semblance of healer balance this game has to offer. AST has been getting mountains of love since its inception in 3.0 and right now I personally feel the toolkit is well beyond the point of being acceptable. If they continue to buff AST in the levels they have been, they'll definitely need to start nerfing other aspects to maintain balance.

I'm going with the assumption at this time that any adjustments they make to AST will be more about minor buffs and adjustments to the kit more than "Hey, let's slap this on this ability / spell and make it SUPER POWERFUL"