Shia will not or say cannot appoint a khalifa.. it is upon God to do that and He has already appointed Imam Mahdi (a.s) as the Imam but he is in occultation after he comes there will be unity every where... He will be the leader.. That will not happen until Allah wishes... No one can say wen he will reappear... And there will be no other leader after him as he is the last Imam... The earth is in existence because of him... If he dies the earth will get destroyed with everything in it... and the last person to die will be Imam (.s)... I hope this helped May Allah Bless you..