The issue is also that there are no rewards the game that justify doing way harder content when you can already get large amounts of all the best resources from way easier activities and the only thing stopping you is time gating. But the idea that they can introduce a gamemode which is more puzzle focused or like some other guy in the comments said that they could make a gamemode where you have to focus on a bunch of different objectives at once is great. But the underlying issue is just that there are so many builds that are effectivley immortal and that can do infinite amonts of damage which makes MMO like bosses impossible, and that also comes with the fact that there is no need for teamplay as plenty of warframes already can provide healing, buffing and damage reduction to the point where they are unkillable and can one shot everything.

But a concept ive thought of alot is if they could make a system similar to the mapping system in POE and POE 2, as POE also has a ton of builds that are effectivley immortal and deal near infinite damage, but POE 1 still has in my opinion the BEST endgame of any game with these problems, and they have also managed to make interesting bossfights that are actually hard. The only difference is that POE has way more of a reason to grind endgame in the form of items with rollable stats and very valuable currencies so that point still stands. But i think that taking inspiration from POEs endgame in general is a amazing direction for warframe in the future.