This kind of exists. The amount of mana you can put into CRD without spending aura is based on the integrity of the spell when you created the cloud. And a higher integrity cast will replace a lower integrity cloud.

From http://forums.play.net/forums/DragonRealms/The%20Traders/Game%20Master%20and%20Official%20Announcements/view/321

>>2) Better scaling! The residue cloud now has a density directly related to the Integrity of the cast that created it. The more dense/higher integrity the cloud has, the more mana you can stuff into Crystal Dart without having to spend aura in adverse conditions. This isn't a 1:1 calculation between Potency and Integrity, but it's pretty generous in my opinion.

>>Like before, you only default to the cloud if the only other option is to spend aura. Unlike before, a higher Integrity cast will override an existing cloud without making you wait until the existing one is gone.

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“I’m sorry that your mystical, godlike powers do not instantly work as you would like them to.”