what topics can be combined with a separation of powers essay?
It is difficult to provide you with concrete advice, as I do not know whether this is at AS, A2 or university level. Neither do you say whether it refers to the UK, the USA or both.
If it refers to the UK, the focus of any question will be on the fact that legislative and executive powers are fused rather than separated, though the judiciary is almost entirely independent. The most common question asks you to evaluate the effectiveness of parliamentary scrutiny of the executive. Another is to comment on the view that parliamentary influence is declining, which raises questions about 'executive dictatorship' (Hailsham) and can link with theories of prime ministerial power. Conventions enter the picture when discussing the role of the Lords, and in particular the Salisbury Convention. Parliamentary privilege has no bearing on the separation of powers. You could also be asked to focus on REFORM of the Commons and/or Lords as a means of improving scrutiny. Similarly, the case for a written constitution could be combined with a question on separation of powers.
In the USA, the Constitution provides for a separation of the powers of the three branches. You can be asked to identify some of the checks and balances specified in the Constitution. Or asked how well Congress carries out its oversight role, especially in circumstances (as between 2001 and 2006) where the same party has control of both chambers and the executive branch. You could also be asked to assess the view that the separation of powers makes strong and effective government impossible. There is also much scope for asking about the role of the US Supreme Court, and establishing the extet to which it can and does frustrate the other governmental branches. this raises the familiar question of whether Supreme Court justices are really 'politicians in disguise'. Think also about 'political parties unite what the Constitution divides' and whether, in practice, powers are really SHARED rather than separated.
I hope this is helpful.