I have been thinking about these more and more. Honestly i think they should be put like towards the end of our priorities, due to difficulty and time it would take.
However if you think about it custom quests or quest in general would draw in a big crowd of people.
They could be quests allowing people to access different areas (my crib if i let go
different items some that stores dont sell or reconfigure the stores so that you can only get the item from beating the quest then you can buy from a quest room or something like that. (id have to think more about that one), different monsters to face, and possibly different skilling places.
Of course none of that would be fun without a bunch of people (well atleast the skilling wouldnt) but this should draw in some people. I would play if we got quests.....
We could start off with an easy quest like going here and there and getting money and what not and working our way up.
After a while you could put a quest guild in offering a quest cape, and taking that out of the shops
I would be more than happy to come up with some plots to the quests and some monologue.