How engaging the open world is, is dependant on the content it contains.
Not by spending 10 minuts running from A coordinates to B coordinates.
Extra traversal methods do not take away from events, quests, puzzles and etc.
They would still be there, you would still need to walk to them, you'd just do it quicker and be less frustrated and/or inconvenienced.
My proposed solution is literally Ardenweald:
Lots of relatively well placed flight paths and an extensive mushroom portal system.
I'd just return the FP whistle and either move around or add some of more Portals/Flight paths. (there is absolutely no need to have a portal just above the Flight master at Glitterfall Basin for example.
Traversing the zone worked perfectly up until flying got introduced and made every travel option irrelevant.
Also helped that, unlike Bastion and Revendreth, it had more likable terrain design.
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You are, of course, free to tell me how shitty Ardenweald really was and how the zone was not engaging at all!
Part of me thinks that i won't believe you tho.