I also really like the quest idea, but I also think there should be special quests for GPs. Nothing is more frustrating than ending up with harbor master as Mongolia.
Of course this also means that this will take more time to develop.
Well that would only be a problem if both a) a GP can trigger several different quests and b) the quest is selected randomly are true. If there are multiple we can let the player choose. If there's just one(or one per era) the Mongolia player would just use that GP for something else as before.
Just spitballing here, but I can imagine it as something like this. A GP offers a selection of three quests, each with two or three rewards to choose from; I also like the idea from the base game that the third reward might require special conditions, like possession of a certain wonder or what kind of civics you run.
Taking a Great General as example:
"Your accomplished military advisor Whatshisface has pledged himself to undertake a great endeavour in your name. You task him to:
1) Turn our army into an elite force: build X barracks.
Rewards
- Our warrior culture has become an integral part of society: barracks get +culture
- Our defenses will become impregnable: Drill I for archery units
- Our attack shall crush all enemies: City Raider I for <best current melee unit>
2) Control the seas: Warships quest
3) Subdue the unruly tribes neighboring on our lands: kill X barbarian/native units
Rewards
- Hold a great triumph to celebrate our victory: +moderate amount of culture in all cities, temporary happiness in all cities
- Use the veterans from this campaign to establish a training facility: military academy in capital(this might need some rethinking, while it would be interesting to be able to earn an academy before the requisite tech it could be rather strong)
- <if not running Egalitarianism> Sell our captives into slavery for profit: gain a large amount of gold, perhaps more if you're running Slavery
in all cities, representing a significant advance in medical knowledge, or a temporary increase of 25% 
for your civ (perhaps lasting for 10 turns?)
bonus equals a Future Tech, or a very powerful wonder(e.g. Notre Dame).
) with working a grassland hamlet by the river(2
3
), you'd see GPPs are not in short supply. Later civs enjoy good GPP modifier bases(80-, no love for ancient civs like China
), and each GP is worth ~2000 flat 
, which is already super powerful. It's important to take these factors into consideration when brainstorming for new GP actions.
