Digital Extremes announced the hard mode for Warframe recently and applications for participation in the test crew are now up and running. Good luck!
Warframe will channel the hard mode through something called The Steel Path. It is meant only for those who have conquered everything on their journey and are looking for a new challenge. By that we mean players who completed the Solar Map will be invited by Tenshin to unlock The Steel Path.
Some of the basic highlights and things to know about The Steel Path include a higher challenge for players with powerful gear "to take on threats at a higher level without having to wait in missions for long periods of time" which should cut down the waiting time considerably.
Furthermore, the Higher Difficulty Series, as Digital Extremes called it, will be a way to get some better scaling Affinity and Mod Rewards. Speaking of rewards, going down the Hard Mode's route will offer the Tenno exclusive cosmetic decorations, emotes and mastery.
DE also included a set of explanations on what the hard mode will not be. Apparently, it's not meant to be associated with endgame or overly complicated in the goal to provide higher-level content and some exclusive rewards. Considering the modern standards of live service games, one could call this an oxymoron since developers are usually content with adding a hard mode and calling it the end game.
Then again, Digital Extremes proved time and again that they don't just conform to the latest trends and if they have something else in mind as content for the nebulous term "endgame", it might be worth it to give them the benefit of the doubt.