If there were a mod that allowed you to play with a mix of two civs (get tech tree and properties of both civs) what combinations would be incredibly powerful?

Off the top of my head, Bulgarians and Poles sounds really good. Winged Hussar with Stirrups and trample damage would be incredible, and Poles provide an economic motivation to mine stone early, which plays greatly to the advantage of the Bulgarian bonuses and techs.

Khmer and Celts could make for ridiculous scorpions and very solid eco.

Slavs and Teutons would have an incredible boom and frighteningly strong champions.