You don't waste the talents of someone like Donald Sumpter so easily, and his reveal as Rassilon made for a great surprise in the Series Nine finale Hell Bent. The last we saw him was when the Doctor stripped him of his powers and banished him. Given his legendary status and his tyrannical nature, expect Rassilon to be out for revenge - perhaps even being responsible for the Twelfth Doctor's regeneration if this really is Capaldi's final series too. Rassilon will like be a shadowy figure pulling the strings behind the latest threat as Series Ten progresses. Perhaps even teaming up with Missy. Given Michelle Gomez's amazing chemistry with Capaldi, expect her to make at least one more appearance under Moffat's tenure and a team up with Rassilon would make for a great villainous double-act.