Recently, Mr. Matsui stated that if there is a demand for more challenging content, then they'll consider adding such content. Let's make the demand known!
Personally, I would love to see more old content be renovated. Unity bringing back old (H)NMs has been fun and nostalgic, but it leaves me wanting something more substantial! As my favorite old endgame events were Limbus and Salvage, I'd like to make suggestions as to how to give those new life.
My proposal is to simply add a hard mode designation to all Limbus and Salvage areas that raises the parameters of the monsters and bosses to be appropriate for 119 Superior players. I feel that they should require a party of at least 6 players, and 119 versions of Omega and Ultima should require an alliance. Of course, the rewards would have to reflect the increased difficulty and increased number of people required! This would mean greatly increased currency drop rates, and powerful equipment. As an example, a 119-appropriate Salvage zone completed fully would ideally drop enough Alexandrite so that every member of a party of 6 could leave with a little more than what they could get soloing an outdated zone.
Although Unity has me on a nostalgia kick, entirely new zones for both Limbus and Salvage would be welcome too! Just so long as any new zones convey the same feelings the old ones did, I'll be satisfied.
Fellow players, whether you agree or disagree with my specific suggestions, if you want to see more challenging content, please post here and make it known! Also, if you have suggestions for what kind of content you'd like to see, post that too!
And to the community reps, please ensure the developers know that there are still many players who wish to see exceptionally challenging content!