In Return to Ravnica all ten gilds will be covered in the first two blocks, so the question is: what will the last block cover? I think the most dramatic thing for a city of Gilds at war to do is simple.
Two gilds forming one to double their power, with the intent of finally destroying the other 8 gilds. There would be plenty of back stabbing and empty promises, but that's half the fun. Here are my piks for the gild "parings";
Marshal Law: Boros/Azorius
They rule this town, from the stinging gutters to the shining roof tops, and the low life's that crawl through the streets have finally gone too far. They will get in line or be destroyed.
Primarily white with blue and red mixed in. They will form the new gild of Justice.
Future Seekers: Izzet/Simic
They fear what we do, what we become. We will give them so much more to be afraid of. No more will they stand in the way of progress.
Primarily blue with red and green, they will become the new gild of Tomorrow.
Immortal Empire: Orzhov/Dimir
This city belongs to us; they just don't know it yet. Their belongings, there family's, there vary thoughts will be ours to do with as we pleas. The sun shall never set on our empire.
Primarily black with white and blue, they will create the gild of Kings.
City Crushers: Gruul/Rakdos
People die, buildings crumble, hope ends. Ravnica has seen its last sun rise. Accept your fate and burn.
Primarily red with black and green, they will found the new gild of Destruction.
Natures Rise: Golgari/Selesnya
The bests have grown to argent. There struggles to fight the natural order will bear no fruit. It is time they returned to the earth.
Primarily Green with black and white, they are now the new gild of the Jungle.
Reading: Atomic Robo, One Piece and other comics