NASA extends Juno, turning spacecraft into an Io, Europa, andGanymede explorer

What other existing, deployed spacecraft can/should have their
missions extended?

"NASA has announced that it will extend the missions for two of its
interplanetary explorers launching during the last decade—the Juno
spacecraft orbiting Jupiter and the InSight lander on the surface of Mars.

The Juno extension means the spacecraft will now operate in the Jovian
system through 2025. This will effectively transform the spacecraft from
a mission to study Jupiter into a full-fledged Jovian system explorer,
complete with close flybys of several of Jupiter's moons as well as its
system of rings.

Back in the inner Solar System on the surface of Mars, the InSight mission
will now run through December 2022. During these additional two years,
the lander will continue to operate its seismometer to identify
Marsquakes, as well as continue to collect detailed information about
weather at the surface.

After determining that both missions had done exceptional science to date,
an independent review panel recommended extensions of both to NASA.
"The Senior Review has validated that these two planetary science missions
are likely to continue to bring new discoveries and produce new questions
about our solar system," said Lori Glaze, director of the planetary science
division at NASA Headquarters in Washington."

See:

https://arstechnica.com/science/2021...n-moon-flybys/