So I had this idea to rework the Kuva Fortress endless missions to roll rivens.
You select a riven at the start of the mission. Each rotation rolls the riven.
The missions will have 10 minute rotations, like arbitrations. Mission rewards will be completely removed (only kuva is rewarded if you dont have a riven equipped) and the only purpose of the missions is to roll rivens.
Once you're happy with the riven's stats you bounce out
If you don't have a riven equipped you receive kuva comparable to a flood (because flood missions are done in 3-5 minutes anyway)