Check out this crossover slide-show. All of the pozz'bilities are within reason - nothing is from outside the 'Doctor Who' domain.
However, I think some of them wouldn't work because the Doctor expressed either suprise or ignorance at his first encounter with those involved.
For example - the Fourth Doctor on board the spaceship Titanic with Leela. He could be on some other ship in the Max Capricorn fleet, but the info-angels seemed to be a new feature when the Tenth Doctor encountered them.
Another example - the Eighth Doctor with Madame Pompadour. As you'll see throughout today's post, I'm a big booster for Eighth Doctor crossovers, but there was never any indication from the Tenth Doctor that he ever met her before.
However, in the cases of the Angels and of Winston Churchill, those are pretty reasonable ideas. In fact, a Weeping Angel might splain away what happened to Ace following the cancellation of the show in 1989.
Number Nine with Syl and Number One with the Master also work. But the Fourth with the animated scarecrow would be right out.
My favorite might be the Sontaran-Rutan skirmish in the woods.....