I'm not sure if the example of RTS games is quite applicable (many RTS games have editors, but few RPGs do, at least outside of modding), but custom dungeons might be fun.
The only direct issue is that they can't offer loot, otherwise people will optimise the fun out of the game and create dungeons that are the most rewarding per time spent within whatever minimum rules are set, ideally with easy pulls as well, which will only make the game boring.
If dungeons can be created purely as challenges to be shared and progressed at rising difficulties, that could be interesting.
However, as good as this sounds on paper, Fellowship is ultimately a mandatory multiplayer game (well, except for the mode they're adding next pre-season, but that's not the core gameplay). As such, I'm concerned about any possible split in the playerbase.
I believe that shelving this idea for future use when the numbers are greater would be more appropriate. More diverse game modes could certainly enhance the long term viability of the game, and ever new challenge dungeons could serve as content lasting long beyond the point when most people have reached their realistic peak for a season.
But for the time being I think they should focus on core game modes and making sure that most queues (even if perhaps not the highest eternal ones) are reasonably short.