Why don't you add different difficulty levels to missions(especially endless missions)? Imagine starting Mot with level 100+ enemies! You would have to balance the rewards and maybe add some extra resources or "endgame" mods that you can get only at higher difficulties, like you did with arbitrations. You could satisfy both the casual and the veteran players at the same time. And to spice things up you could use the different effects that sorties have(energy reduction, radiation etc) and add them to some of those higher level missions at random resetting each day.
Just imagine the possibilities!
Another suggestion would be looking back at the cetus and fortuna bounties. Bounties are great but they lack two things: Viariety and higher difficulty scaling. Right now they are a joke to do even with unranked weapons.
In general, try to focus on making the existing game better and the rewards feel more satisfying(i'm looking at you kuva disruption). Railjack seems like a nice addition to the game but i'm afraid it will suffer from the same problems that i mentioned above. It will be a nice distraction for a time but after a while we will be back at point 0, waiting for the next "big thing". This type of content is a nice dessert but not the main dish. If the main dish is bad then you are looking forward to the dessert. But if the main dish is good, by they end you will be full and you won't even care about the dessert.
A lone tenno operative 🙂