Obviously, the constitution, but i'd suggest federalism would be a pretty major early issue.
If the two blocs are Right and Left, then social/welfare and economic policy.
I've suggested in the past we establish things so that we already have a European, even post-Communist economic system with the far left wishing to maintain it, the moderate left and right wishing it to be 'preened' through market forces, but retaining free health etc and the far right (economically) wishing to abolish it.
That gives not only a right v left battleground, but also some internal divisions too.
