I've been playing DDO since shortly after it became free to play. I really enjoy a lot of aspects of it, but I often wish there was more variety in quests. In fact when I was a newer player the lack of variety almost made me quit as I didn't like having to run the same content over and over in order to progress.
So as the title suggests, I think giving players access to tools to design quests and generate content would add a ton of interesting content with the benefit of making DDO live up to the customizable nature of its namesake(D&D). In addition, it would be a great opportunity for developers to optimize tools they use for quest creation so they can create content more rapidly in the future.
Obviously the implementation would need to be structured in a way that promotes fun cohesive quests and prevents quest designs that are exploitable, incoherent, or offensive.
I'd suggest having a single entry point that leads to any quest/public area that has been registered. It would be made clear at the entrance that the content is player submitted.
Registering a quest would cost a decent amount of DDO points and would only last a set period of time, considering data storage and server costs something like 1000~2500 ddo points for 1 month time should strike a decent balance and prevent players from submitting half-baked quests. Perhaps even cost being based on the data size would add encouragement to optimize quest performance.
Moderator(s) may be necessary to ensure no offensive content is registered. There are 3 major options here, player volunteer moderators, paid staff, and/or public reporting after content has been registered.
Quest rewards would be severely limited while at this player submitted stage. DDO staff could monitor player interest/rating of quests and decide to turn select content into official content, and in doing so polish the quests and add balanced rewards.
All in all, I think this would breath some new life into DDO and allow for rapidly expanding content in an affordable way.