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".
It doesn’t have to be game breaking. Like most things in 5e flight is typically a scant resource, at least until very high levels. I’m playing Descent into Avernus with a party of five level 12 PCs right now and the only one who can fly in our party is the artificer, with winged boots he used an infusion slot on – so he can fly four hours a day. Most of the fiends we’re fighting can fly so they have the advantage. The fly spell is third level and only lasts for ten minutes so it’s a big call if it gets used as slots are valuable and resting isn’t ridiculously easy like in BG3.
Sure, some playable races can fly… if you want to make a whole party of aarakocra then that’s on you (if BG3 will even add this race). My main point is that if there is no flight mechanic it shifts the whole premise of the game further away from D&D, no flying monsters (it’s dungeons and DRAGONS after all), no elevation on spells, a bunch of items won’t exist. And as others have pointed out, Solasta has pulled it off - albeit not perfectly but they have a studio of 17 devs IIRC.