What about if:
It's just after Let's Kill Hitler and now that the excitement of meeting the Doctor is over, River is just a little bit terrified. Yes, she's going to be very important to the Doctor, but that's not yet. That's in the future.
And now, the only thing she can think about is the fact that she's just given up all of her regenerations to save a man she's been trained and conditioned into killing since she was born. She's failed. And now her best friend knows she's actually her mother, which is all extremely confusing.
Then she meets the Master, and he's every bit as brilliant and amazing as the Doctor is. Every bit as wonderful. But he's better, because whilst he's looking for the Doctor and seems to like him, he also understands her and wants to make things work for her.
Maybe, on their path to finding the Doctor, they discover a plot in the past that has something to do with something, and all hell breaks loose.
The Doctor, naturally, turns up just in the knick of time to help out - there's a young man called Arthur who has a secret. He was the last King of Camelot's only son. He knows he should be king, but he doesn't know how to rule, so he shies away from it.
Enter the Doctor, who realises that if Arthur never becomes the King of Albion, never unites Britain, then the modern Western world as we know it would fall, and everything he's worked so hard at would be for nought.
Enter the Master, who apparently quite likes the fact that modern day society is perfect for him to mess up - he doesn't want to completely change a planet's history! Or at least, that's what he says. So he and the Doctor work quite well together, which surprises - and pleases - the Doctor, as he's always hoped the Master could be redeemed, but never thought it possible.
Then a whole load of plot happens, and by the end of it, the world is saved - huzzah! - and Arthur is crowned King of all Albion. And the Master's side project - a little science freak called Mordred, of all things - is left to grow up in peace.
Until, of course, the day Mordred finally kills Arthur, and the world is shoved back into chaos, but at least now it's historically accurate chaos, no?
And the Doctor and the Master become immortalised in human history as Merlin - that weird, old guy who helped Arthur, and also really liked birds - and Morgan Le Fey - a man somewhat obsessed with Merlin, but who tried everything possible to disrupt Arthur and lead to his downfall.
That's all I've got, lol.