All the current advantages of the EU but better organised and being a significant force in the world.
I think most of Europe has already tried the significant force part.
I think as part of the brexit deal we should allow all those in Britain who wish to be part of the eu to go and live in Europe.
States aren't powerless within a federal state - what powers they have depend on the constitution. I'm sure Scotland feels powerless within the UK at the moment, so should it be separate? What about London, Yorkshire, or Birmingham? Why is the UK the entity that should have this "power" that you speak of?
Read it in our history but as far as our union goes I would have no problem with Scotland Wales or Northern Ireland leaving, I don't believe they would survive on their own though.
As for the federal state part, I haven't seen a good working model yet.
It was academic anyway because, as I said, Cameron had negotiated an opt-out. In other words, the EU were going to amend the relevant treaties to make it all but impossible for the whole EU to become a federal state.