I haven't worked on modding before and dont have much motivation to start up modding minecraft now but I had an idea that I would like to share to any modpack creators that are looking for inspiration.
I play a lot of modpacks like regrowth, skyblock/factory, project ozone and now stoneblock. My idea would be very similar to these packs but instead of starting on a sky island or underground you would start at the bottom of a world completely covered in ocean inside a dome. the first thing that came to mind was the rftools shield generator and how it could be used to create a dome around the player, then the idea of quests that push the player into creating more ways to generate power to increase the size of this shield/dome so they have a larger area to work in. possibly a quest chain on creating rft dimension worlds to gather materials or based around mystical crops/ botania like regrowth was.