I'd have loved to have seen Simm with Smith, but unless they have him in the Xmas special which I doubt, that'll never happen.
Maybe Simm and Capaldi together would work. Simm would comment how old he is and just insult him all the time.
If they recast, maybe Philip Glenister [b]could[b] work as an opposite for Capaldi. I could just imagine them telling each other to fuck off all the time
But seriously, Philip could work, Benedict Cumberbatch could work as the Master, although he'd have been more suitable for Smiths Doctor, and I think he'd be much better as Omega, um.....