I very much enjoy the randomization of the Purple Cave Quest, and I had one wonderful "WTF" moment thus far when doing it, so I think other lower and higher level quests could benefit from a similar system.
A few quests that come to mind:
The Wildling Tribe Quest (Where you can find all manner of wildling associated loot) could be randomized to create a slightly random tunnel system, with (as the Purple Cave) different enemies and different loot opportunities.
The Exploring the Trash Heap quest (found right next to the Elevator) could also have random setups, for better or worse.
Another quest that could be made VERY interesting this way is the relatively boring Hook Horror "Rescue the Recruits" quest. Maybe Spiders took the Watchers? Maybe a group of bandits? Perhaps a small goblin tribe? Or perhaps a Necromancer killed the Recruits and turned them into his loyal minions? This would put a lot of life into this quest. People would stop saying "Let's go kill Hook Horrors" and instead could say, "A group of civilians went missing, let's investigate."
This randomization would make it difficult for people to prepare for these quests, especially if the veriety of possible enemies would be large enough, but it would be so much more interesting!
I guess the Ibrandulin quest could be randomized too! What has taken the faithful? Is he fine? Does he need help? Has he got married? A Dendar Hag devoured him? etc. etc.
It could help with even more lore regarding Ibrandul's faith.
I'd say very strongly the Kobolds, Goblins, and Gnolls quests as well. Make quest grinding a bit more dangerous for the chronic questers, and more fun for villains who have to keep their level up.
Randomizing quests takes a lot of work but as I can well imagine a lot of these quests are starting to feel pretty stale I might try to look into it.
I wouldn't say that the quests are stale, but I would like to put emphasis on how interesting the new design is. Maybe instead of adding new quests, it could be nice to start making variations to the existent.
One of the things I've been meaning to do is hide particularly rewarding scripted quests within Exploration Areas.