Admittedly, the last time I stuck around after a clear was ARR AK to do a Chest run since everyone avoided the chests during the clear, I don't remember being so limited on time back then. But things change I guess.
I could see so many crafter/gather opportunities with having dungeons be explorable. Bayflox longstop and hauke manor I always felt should just be an open part of the world. It's fairly open looking as it is. Any dungeon after clearing the hard mode should be opened up for casual exploration I thought once upon a time. Newer dungeons are a lot more linear than the ARR ones tho so it's hard to really open them up.
I don't understand why diadem is instanced... Just make it an open zone already since it's never used. Would get more traffic I bet.
You just run a dungeon unsynced if you want to take yur time and explore. And with newer dungeons, there is an 80 minute time limit and most of the time, it takes about 20 minutes or less to clear, most of them allow you to back track.