In my opinion, Cromwell should have just pulled a Napoleon and declared himself King. We can all see what happened when Charles I was overthrown and the country was about to descend into chaos. It took a man with the strength of character of Cromwell to restore law and order, revive the disintegrating power of the state and ensure that lives could return to normality. Cromwell's suppression of the trouble-making Levellers and other such dissenters helped to ensure a climate of peace and moderate prosperity, and it is a shame he died before he could see his reforms to their logical conclusion. Cromwell's death led to near-anarchy before the ruling class saw little choice but to bring the King back. Cromwell may as well have declared himself king.
Cromwell basically made himself king in all but name, he declared his son to be his hier and everything.
Henry VIII should have declared himself Pope like a boss.