7. "Mission to the Unknown"
"Mission to the Unknown" is an interesting example. On the one hand, it's only one episode long, so it wouldn't take too long to animate. On the other hand, it's already been recreated, albeit in a completely different way from animation, or even photo reconstruction. Back in 2019, a new live version of the episode was created with a completely new cast.
If you haven't watched the 2019 recreation of "Mission to the Unknown" yet, I highly recommend checking it out. Making a new version of a missing episode with a new cast isn't something I'd usually be in favor of. But "Mission to the Unknown" is something of an anomaly, as it doesn't feature any of the TARDIS crew.
Instead, it focuses on three brand-new characters: Marc Cory, Gordon Lowery and Jeff Garvey. They've crashed on the planet Kembel and have to repair their ship, all the while fighting off the venomous Varga plants. But one of them is on Kembel for a reason - because the Daleks are on Kembel too...
You could argue that there's less incentive to animate this missing episode, especially with the 2019 recreation already existing. It's an incredibly faithful recreation too, at least in terms of production, looking like it was made straight out of 1965.
That's not to say that there's no chance that the original version won't be animated. Especially due to where "Mission of the Unknown" leads to. Because while this story is only one episode, it's really a prelude to something much, much bigger...