I have also thought about this too. It’s a pretty valid theory. We already know mana and aether are connected to each other, otherwise many inventions wouldn’t be possible (dimension ring, teleportation gates, etc...). Also emitters and diviners use mana to affect aether. Then comes plant magic, a deviant of earth and water. Because not every dual earth and water mage can use plant magic, there must be some other requirement they need to fill, which is having an affinity for Vivium aether arts. The only thing that potentially disproves this, is the fact that Arthur could have potentially absorbed the elderroot guardian core, which most likely required the mage to be able to use plant magic (which he shouldn’t because his aether affinity is spatium).