Personally, I'd be fine with using the weaker ones more than once, as long as they're used very rarely, so it does still give that Council Member feel Tormentor mentioned.
What about having sort of Sons and one Head of the "theme" for each episode?
For example the more you proceed to the end map of one episode, the more Sons of X you will encounter. At the very end, expect a powerful master of the sons as boss battle.
And at the very end of the game, 4 masters against you in an epic boss battle in the void