But the ISS should be considered a manned instrument bus. Consider OCO-3, a gimballed observer on the exterior of the ISS. It has revolutionized our understanding of global carbon cycles, and local carbon sink/sources.
There are numerous such missions that benefit from a common, well supported platform to operate from.
The next one should be robotic though.
https://en.m.wikipedia.org/wiki/Orbiting_Carbon_Observatory_...