There are myriad quantum possibilities, whole universes existing just a molecular vibration away from our own, worlds where things are very different and others where they're almost the same. In what we call "Whoniverse-B", the Doctor's adventures are just a touch different...
When the Doctors teamed up to fight Omega in Whoniverse-B, the first Doctor wasn't trapped in a time eddy and could join the others.
That universe's Mr. Sin had a Mini-Sin sidekick, a homonculous created from a 19th century pig.
And there was the time Ace found romance on the Planet of the Cheetah People.
The 8th Doctor not only gave his TARDIS a makeover as soon as he was able, but was also tasked with assembling the Key to Time AGAIN.
Reeling from the Time War, the Dalek Emperor's new Daleks weren't quite up to snuff. The dustbin nickname/insult was never so appropriate.
As more and more evidence of Whoniverse-B pours out of my own personal dimensional rift, you better believe I'll get it out to you!