Not sure how I feel about your proposed solution, but I agree it's a giant boost to have a flying mount. I'm actually in the camp that flying should be obtainable by late-game players with serious effort. It's too cool of a thing to not do in a world like this. I think that there should be limits on how long someone can fly, however, as I do understand the ridiculous power unchecked flying grants players in open world games like these.

I think there should be a daily limit to flying and strict no-fly areas. This could thematically be tied to mount stamina possibly? Players should need to think about what they're going to use their flight time on every day. Do they want to be ready for a world event? Do they use it for efficiency with their gathering? Perhaps there's a war happening a couple nodes over that the player simply wants to check out as a third party? Maybe there's days where players used their flight stamina for something mildly beneficial only to require it later for a surprise event and not have it. This is in line with the risk vs. reward philosophy that AoC is going for I think.

Would be cool to hear other's thoughts, maybe others don't want it at all? Personally though, I think flying is cool and I think it should be available to players who want to work for it so long as it's not allowed to take over the meta in crazy ways. There's also the idea that this is inevitable, which I think it is. It may not be in at launch but I think when the game starts expanding, flying mounts are going to be something players want as they grow accustomed with other features.