Unique Companions you'd like to see.

[A Child of Gallifrey]

I am definitely for the humanoid TARDIS making another appearance. That would be awesome, but the TARDIS would have to inhabit a different body.

[Faith Charity]

An Alien whose hair changes colour. Also someone who happens to get stuck within the TARDIS and cannot leave

[LegionPothIX]

I think that it would be really nice to see Madame Vastra+Jenny, as a long term companion (for a season or two) instead of the possible episode hinted at by the battle of demons run. I think it'd be a great way to show a non-traditional relationship evolve that isn't just the red-head pushing around the dopey blond (for a few thousand years), that doctor who is so fond of using as relationship standards.
The two showed a certain level of familiality and chemistry as characters, and I'm sure it'd get the ratings they're looking for on the show.