Would/should they CGI in the deceased actors?
You could probably pull elements from the various movies or from the original series now that we have both in HD to create a nice effect without having to go CGI. (I assume you mean creating CGI replicas) Just create a digital composite, maybe add in some motion into it.
My vote would be NuTrek viewing TOS era counterparts. Have the ensemble cast look at the prime versions of themselves at the same approximate age.
But I also think that for the 50th anniversary, they should find a way to squeeze Shatner in for a real role.