Fantasy Alter Egos/ alternative Forms of the Doctor

Transformers Doctor Who (categorically not that rather predictable “transforming police box” or any allusion to Optimus Prime). I’m thinking either something like Sixshot with tonnes of alt-modes or just the one, exploration orientated mode (like Cosmos).

Yeap.

Why do they have to look like a human/Gallifreyan when they regenerates? I’ve already regenerated the Doctor into a Silurian, so why not a living robot? 
And Maximal/Predacon era Transformers are “technorganic” beings.
Maybe the Time Lords exiled the third Doctor to Cybertron rather than Earth and slotted him into a robot body rather than a quintessentially British Englishman.

There's a precedent for that, too. One of the theories of the renewal process is that part of the "material" for the Change comes from the surrounding environment, so if the Doctor were to regenerate on a biomechanoid world...

Part of my headcanon, currently, is the reason why we tend to mill about the first 100 centuries or so is because once you get into the 200th+ century, you start bumping into incarnations that haven't happened yet. I do like Craig Hinton's idea that the Doctor's final form before permanent non-existence, however many millennia and universes away that may be, is the Guardian of Justice.

Well, the theory goes that alongside the Black Guardian (who mediates universal chaos) and the White Guardian (who mediates cosmic order), there are other such personifications which embody different parts of our universe.

In total, perhaps, are the Azure Guardian of Balance, the Silver Guardian of Space and Matter, the Crystal Guardian of Chance, the Gold Guardian of Life, and the Red Guardian of Justice. All who mediate various aspects of what the Doctor sometimes referred to as Creation. Essentially, the embodiment of everything. Everything that was everything, including the nothing in between. I like the idea that, at the end of their travels, the Doctor may cease to be, but in doing so, becomes a universal constant that guarantees someone will step into the gap and try to make things just.

Because if you think about all those other Guardians and their role -- even Life and Balance -- there is still no guarantee in the universe that things will be fair. To have that, feels like the ultimate victory on the Doctor's part. (This is long after we'd be capable of following them, naturally. Beyond the event horizon of their lifetimes. At the, I suppose you could call it, the end of ends.)