Disregarding the question of whether the states of Europe are ready to join into a single federation, I would venture to say that there is no way to write a constitution or anything else that will prevent these three of your listed problems:

• How will this text prevent Europe from engaging in imperialist wars?

• How will this text ensure that checks and balances are enforced upon all four branches of government?

• How will this text handle secession?

The first two are virtually unavoidable because of human nature. And if you allow secession, then you hand a big stick to the states that they will try to use at every opportunity.

But the real stumbling block is that Europe does not yet see an advantage to forming this sort of union. Whether the current setup will collapse is not determined yet, but one possibility is the failure of the Euro system given wide disparities in economic conditions across the continent, and another is unresolvable problems with border control...