Okay, here's
threefour to start.
Venice: "Enrico Dandolo, you are poised on the doorstep of a pivotal moment in the history of your Republic. For centuries, your Venetians have used maritime skill and control of trade routes to become the richest nation in Europe. They continued to build on this as three great Crusades left for the Holy Land, largely sailing on fleets that you provided. Now a Fourth Crusade is gathering- but this time, they aren't able to pay their way.
Now is your chance to become the greatest Doge in Venetian history. The Crusaders have agreed to buy their passage by doing Venice a small favor. They can return rebellious Zara to Venetian control- but your plans are much grander than that. The former colony is about to triumph over their master..."
I figured I'd mention Dandolo (instead of "you") just because the timing works. And on the flip side...
Byzantines: "Emperor. The great Eastern Roman Empire continues to flourish, with both land and riches- and Constantinople remains the crown jewel of Europe. However, we are but a shell of our massive former glory. Bulgarians, Arabians, and barbarians of all sorts have swept over Anatolia, the Levant, and southeastern Europe in great waves.
Now we fear we may have our greatest challenge yet. It appears that the western Catholics, still upset over the Schism, are preparing another destructive Crusade. Not to pillage their way across Anatolia as before- they, specifically our former colony in Ravenna, are aiming for our beating heart, the great city of Constantinople.
Our walls have never been vanquished, and many have died trying. But this test may prove too much. And if the Venetians can bring them down, how will we fend off uprisings from the heathens in Anatolia in years to come?"
(I can't remember the Byz UHVs off the top of my head, so I didn't mention them. Is three paragraphs too long?)
England: "The 13th Century dawns over an England in turmoil. The Lion Heart lies dead, and the nobles are unimpressed with his brother John. The soldiers have mostly returned from the Third Crusade, dejected that they never even tried to attack Jerusalem. They face a growing threat from Scotland in the north, a strengthening France on the Continent, and dissension amongst their own barons.
You must lead your English people through the looming civil wars and emerge stronger than ever. You must soon marshal your forces against the French, as Normandy and Anjou depend on your protection. With technological prowess, industrial might, and your strong leadership, you will turn this island nation into an empire on which the sun never sets!"
Too cheesy, with the "sun never sets" stuff? 
Also, I wasn't sure about "Anjou", I'm pretty sure that's not even a province, but I don't know (yet) what all England controls at the beginning of the scenario.
Arabia: "The great Saladin has gone to Paradise, after rolling back a century of Christian gains in [what do Arabians/Muslims call the Levant/Holy Land?]. The holy city of al-Quds is now back in Arabian hands, and the "Crusader States" that murdered Muslims by the thousands are all but eradicated.
You have been chosen as the new ruler of the Ayyubid Caliphate, and must now continue the jihad against the infidel invaders. There may be more Crusades to come, although we have reason to believe we'll be spared from the next one. Other Muslim nations will rise to help us in our holy mission to spread Islam across this continent!"
I've never played as the Arabians or the Ottomans, so I don't know how much they help or hate each other, so that last sentence might be all wrong.
I'll try to look at it tonight, but no promises.
