It's a interesting concept, but I have a hard time seeing this implemented in a game such as FFXIV. Partly because when you say

"Hey this is kind of how a DEAD MMO did things."

Also players these days want a reward for doing...well anything it seems they can hardly walk from point A-B without crying if someone doesn't hand them a gold crown at the end of the journey across the hall. So such a "Custom quest system" would need a reward system in place to be used...and in such fashion players would ignore the entire "Point" of such a system and just find a way to short-cut the whole thing for the best reward.

Tough balancing act, it seems like a feature you would need to work on from the get-go to integrate into the game foundation and as such I doubt we will see it in XIV.