this game has no satisfing end-game content. And every year they simply add more guns, swords, warframes and new mod systems to make your character stronger. Basically if you are maxed out, all you do is wait for the developers to add a little extra from time to time.
But I'm not maxxed out and I'll never be, because this game is simply a crazy grind/farmfest.
The only joy I had in this game was teaming up with good players and doing long farming runs... but even that gets old really quick.