An agonist. While an antagonist blocks an action, the agonist causes it to happen. Even more than a muse, a professional agonist might be exactly what you need to provoke your best work.
And of course, a procrastinatrix. Someone who's only job is to hold you accountable for getting it done, now, not later.
In a world with fewer bosses than ever, when we are our own boss, these two functions are more important than ever. If you can't find a way to do it for yourself, spend the time and the money to find someone to do it for you. Neither job is particularly difficult to do, but it's hard to do to yourself. Two more job titles for the future…
[Thanks to Sunny for the nomenclature.]