Doctor Who's TARDIS has unrealized potential. As any Doctor Who fan knows, the Doctor's TARDIS is a battered old Type 40. She's rather temperamental, and the Doctor has always struggled to get her to go where he wants her to. Matt Smith's Doctor learned why, when the TARDIS was briefly given a corporeal form, and she explained she took him where he needed to go, not where he wanted to go.
TARDISes are dimensionally transcendental, bigger on the inside than the outside. Their exterior is masked by something called a Chameleon Circuit, which allows them to hide in plain sight. A TARDIS with a functioning Chameleon Circuit can be disguised as almost anything. The Doctor's TARDIS, of course, does not have a working Chameleon Circuit; it broke down when he was briefly staying in London in 1963. The Doctor has occasionally tried to fix it, but in truth she's rather come to like the distinctive police box shape. Of course, from an out-of-universe perspective, the broken Chameleon Circuit neatly explains why the BBC won't have to constantly commission new TARDIS props.
Curiously enough, precious few other TARDISes have actually been seen in Doctor Who; the Time Lords tended to keep to themselves on Gallifrey, so only a few Renegades have been out there traveling through time and space. But with only a few exceptions, these Renegades have always left their TARDISes in fairly traditional shapes. Here are some of the typical TARDIS disguises:
- Cupboards
- Grandfather clocks
- Fireplaces
- Columns
Only one Time Lord has ever been particularly creative with his TARDIS - the Master.
The Master has followed the traditional approach on many occasions, but he's also concealed his TARDIS in more unusual forms. He's been known to hide his TARDIS in the shape of a building, ensuring it blends into the landscape, or even as a statue on display. More interestingly, though, twice the Master chose to conceal his TARDIS by turning it into an actual vehicle. Back in the Jon Pertwee era, the Master's TARDIS was disguised as a horse-box, and one could actually climb in the driving seat and drive it away, unaware the back doors would open up to allow entry to a TARDIS. Later, during the Peter Davison era, the Master actually concealed his TARDIS as a Concorde airplane. Ironically, this one probably wasn't a good idea, given the Doctor was investigating the mystery of a Concorde jet being stolen through time. He swiftly guessed what it was.
But these examples point to the fact the true potential of the TARDIS has never been realized. While the Doctor's TARDIS is stuck in the shape of a police box, other TARDISes could be disguised as anything. Any house, any building, any statue, any fireplace, even any vehicle could in reality be a TARDIS. Indeed, given TARDISes are dimensionally transcendental and their doorways are portals as much as anything else, there's no reason a Time Lord couldn't take things still further. Imagine a Time Lord wearing his TARDIS as a wrist-watch, opening a portal when he wound it up. Indeed, such smart approaches would dovetail perfectly with the Chameleon Arch, used to implant false memories in a Time Lord so they can integrate with another planet's society. "Sleeper agent" Time Lords could easily be carrying TARDISes in their pockets, completely unaware what they had in their possession. The potential of Doctor Who's TARDISes is far greater than viewers have realized - or than showrunners and writers have demonstrated.