Inspired by https://paragonwiki.com/wiki/Mission_Architect from City of Heroes!
The GW2 idea- People jump into the Fractals of The Mists and create their own fractals for the player base to play on! Rewards could be 1/2 of the normal fractals to lessen farming. Rewards are the normal ones you find in Fractals. Whatever other restrictions... (below is what the Paragon Team used for CoH)
"Restricted Rewards in MA
Only Architect Entertainment Badges can be obtained in Architect Entertainment missions. This means no critter defeats count for Defeat Badges, Achievement Badges do not increment, etc. Even if a Story Arc gets promoted to Dev's Choice or Hall of Fame, these badges are still unavailable through the Mission Architect missions.
The way the custom foes were designed and the limitation on arcs (like the inability to place foes exactly where one would like) are designed to prevent the Mission Architect from becoming a Power Leveling tool. Changes have been made several times to curb the use of exploits and bugs to gain extra rewards, including altering enemies to give different rewards within Architect versus without, changing or removing maps, and other ways."
...Currently there isn't really anything the players can create and share with other players, and I think something like this would be of benefit to the game and community.