Flying would be extremely abusive to the game in its current iteration and almost has no real advantage to the game itself. the entire reason most people want flying in games is to ignore the terrain, which basically would invalidate the entire games size, and means of exploration. Adding flying to the game would be extremely abusive to the game, and very determinal to the over all experience. I am extremely against the idea of adding it to bg3, or even after working on wow for so long any other game.
That being said, as a seasoned game designer, I recognize that it is one of two universally unique and trans-genre game concepts that are considering "fun" (the other is extreme speed).I'd be more willing to upgrade the value of things like movement speed skills, or even adding something that increases movement speed out of combat. adding flying to bg3 would make a large, explorative form of game play feel like we have no room for anything, would result in massive amounts of "min-maxing routes" to get all the "Secrets and goodies", and would ultimately end with mass complaints of "no content" because of significantly (and I express that with a serious tone) increasing player progressing rates.
I cannot express how bad this would be for the game. Like "it could kill it and the company, bad".