So this one may be a stretch...
Your challenge, should you choose to accept it, is to have a Russian Tsar married to a member of the British (or English if it's earlier) line of succession and the unlikely event that that Briton should inherit the throne occurs, thrusting Russia and the UK into a personal union which their heir officially makes into a lasting political Union.
Also...
What are the effects if there's any possibility?
Unlimited dates on PODs.