People play mainly for the PvP, PvM and skilling.  Small number of players are only questing if they cant afford the gp unlocks for gear or they genuinely enjoy the questing (albeit with bugs here and there).  While the unlocks provide a GP sink, it devalues a lot of other items/achievements.  For example, the dsq left half is ridiculously cheap for a rare item because the majority of the gp is going not to the player upon getting this rare drop but instead to the game (i.e. when buying the unlock or the right half).  This is just one example. 

My suggestion is a new tradeable item 'quest completion certificate' be added to monster drop tables (all monsters or normally killed monsters) which would allow players to hand it into a new npc (in edge) to pick which quest they want to complete.  There could be different tiers like easy, medium, hard, elite, master certificates where the quests  are grouped into these tiers (based on their reqs, lengths or rewards) and the lower tiers are dropped by lower monsters while higher tiers dropped by higher monsters.  For example:

Easy - Man
Medium - Giant
Hard - Greater demon
Elite - Fire giant
Master - Black dragon, KBD

Lets say I kill a man, I can give this to the npc and pick 1 of x number of categorized easy quests (e.g. cooks assistant) to be automatically completed.  Alternatively I can sell it to other players.  Hence, an easy certificate may be tradeable for perhaps a few thousand gp, while the master certificates (to complete underground or legends) may go for a few hundred thousand or even a few mil.  There could also be a small fee charged by the npc when handing in the certificate so as to provide a gold sink.  The drop rate could be tweaked over time so as to ensure they are not continually farmed, but ultimately it provides both the player and the game with opportunity to benefit.