I'll probably go with Act 5. My bro will probably pick up the expansion and, seeing how he played ahead with his friends, getting to lvl60 within two weeks of launch, while I slowly played through with all 5 classes, this will be our way of getting a fresh co-op campaign out of D3.
Will probably run with my Wiz first to get her up to lvl70 and loot run capable.
From here, I may level up another class to check out new skills, or I might start that Crusader. I've been having fun building my second monk with a Vit Hellfire Ring so a new class with the same godly tanking ability will be quite fun methinks.