Both Imu and Blackbeard will be a final villian, in which order is anyone's guess but does that matter?
Imu represents the opposing force of the World Government which is the barricade of the end game. Blackbeard represents the force of opposing ideals of pirates chasing dreams.
Blackbeard's demise will be the fact he didn't put effort into his crew and bonds. By the end of the series Luffy will have accumulated so much knowledge and strength through his bonds that it will ultimately become Blackbeard's fatal flaw. He will lose due to something he lacked, I can't see Luffy winning with only strength.
Ultimately Luffy may have to face Blackbeard for role of Pirate King, however for the fufillment of what was told at Laugh Tale and end game I think that Imu will be the opposing final force.