For new content, what if you gave both new players and the Vets a chance to be a Elite Character(s) for the other Factions. Give player a chance to play as a Elite Stalker, a Corpus, a Grineer or one of the other Factions. A Elite Infested Stalker or whatever. But have them play through the game trying to stop and undo all the Good that the Tenno accomplished. Something like what you're doing with the Emissary.
This will also allow players to have the option to battle one another, if set settings allow it and doing normal game play missions. Imagine, you're doing a mission and a player invaded your game. Now you have to both completely both your mission and try to defeat your invader(s). Have it set up, that you can control how many people can invade your mission(s) or if they need to be in a Squad to invade your game. Cause you're in a Squad. At the end of each missions which ever player or team is defeated. You receive a Blueprint(s) for either the components for there Frame, to built your own version of that Frame or a Blueprint(s) for one their Faction greatest Weapon(s) or Item(s). And you receive a Blueprint(s) from each opposing player.
So the Tenno objective is to complete their mission and defeat any new opposing threats. While the Player from (let say) Grineer Faction objective is to undo any progress the Tenno completed and defeat them and whoever threatened their Faction Plans. Since their now so many Planets to start from. Each Factions would have there own start planet. Players new and Vets, when logging into to the game, will now have the option to continue playing with existing Tenno Warframe or pick as a new Faction Character. But will be allowed to switch back and forth between Tenno and the Factions. This will create more Missions, Weapons, Frames, Gears, Cosmetics, and a ridiculous amount of more Content. I wouldn't mind joining the Crew (if they would have me), to help design all the new Elite Factions Characters. Each Faction can start off with 4 Characters each and over time more can be added to the mix. Just like the Tennos, you can unlock, buy and build the other Frames or the other playable Characters in your Faction.
Don't get me wrong, the game is already Amazing. This type of content will increase the number of players and insure the Future of Warframe. I enjoy the game, keep up the great work. Thank you.