Here's my thoughts, for what they're worth.
At the back, we need to go with the strongest defence we have - which is, unquestionably, Raven, Warren, Meekings and Shinnie. Warren and Meekings are better players than Devine, and an established partnership too. Frankly, Shinnie is better than Tremarco at everything.
Draper must play in centre mid. We badly miss his physical presence, all the more so when Foran is out. I'd much rather have Tansey than Polworth, again because he is a better player. Polworth might yet come good, but when he plays poorly (such as in Perth or in Kilmarnock), we are drastically weakened in an important area of the pitch. If Vincent is fit, then slot him back into his role between midfield and attack, where his workrate and engine have proven to be far more effective for us this season than Nick Ross' attempts to impersonate Andrew Shinnie.
On the right, I tend towards Doran rather than Watkins, simply because the Doran-Raven partnership works so well and Doran is never as good when on the left. The downside is, as RiG pointed out, Watkins offers us an effective diagonal because he's good in the air. Maybe we could use Marley on the left, but I don't recall him playing there before and a cup semi final doesn't seem like the best place to experiment. So maybe Ross on the left, where he will cut inside and open up space for Shinnie to attack? But if we went with Williams' pace, then I wouldn't be particularly upset.
And up front, Mckay, of course. Who else?
So, in conclusion, it should be: Brill, Raven, Warren, Meekings, Shinnie, Draper, Tansey, Doran, Vincent, Ross, Mckay. Subs: Esson, Devine, Tremarco, Watkins, Williams