We’ve got characters with aether sickness, conjurers drawing upon their own aether when they fail to borrow it from the Elementals and slowly killing themselves, ShB has told us that one’s aether can be skewed off balance toward any element, we have the Garleans who for whatever reason can’t manipulate aether which afaik hasn’t been explained yet, we have people with ties to Voidsent who must consume the aether of others a la the RDM questline, Lightwardens who are half-human and retain their sentience… those are all canon, lore-abiding things.
Non-confirmed things that are plausible or at the very least not super lore-bendy, Mary Sue-ish, OP, or otherwise “bad” are people RPing characters either tainted by the Void or the Light, characters that have returned from the dead, or characters that have otherwise had their aether affected in some way because hey, this a FF game (heck, I’ve seen people RPing Primals). Your aether being off kilter is bad for you and potentially the others around you, but it won’t necessarily or immediately kill you.
As for “seeing” aether, I’m sure people can’t actually, literally “see” it without special circumstances like Y’shtola’s condition and the Scion’s aether viewing devices (though this is RP–is it really unlikely or “wrong” for people to have characters with special circumstances like that?), but it seems plausible (if not necessary) to me that some mages would need to have a sense for it. They can literally manipulate aether but can’t sense it in some way? How would that work? I think “odd aether” is an RP hook and a courtesy for those who decided to give their character some sense for aether, as many have, whether you agree with the lore-friendliness of it or not, to offer them the knowledge their character would have about your character.
At the end of the day, this is RP. People are gonna fill in the gaps where we don’t have knowledge, they are going to correct or ignore the parts of the lore that don’t make a lot of sense, and some people are going to straight up add in their own rules to the world or ignore the ones they don’t like because that’s what they want to RP and there’s not inherently any harm in that. ¯\_(ツ)_/¯
