Stralman wants 4 years or more, so he's out.
Niskanen wants a long-term deal and will likely cost too much, plus he hasn't proven a lot, so he's likely out.
After that, it's Diaz, Fayne, Gilbert and Flood, and other slim pickings.
Boyle is really the perfect stop-gap. He won't want more than a 2 year deal, he's a veteran that brings experience (other than Kronwall and to a lesser extent, Ericsson, the D is very young). You saw how long it took Smith to adapt, think about it when Sproul/Ouellet are expected to play big minutes if the Red Wings don't sign a RHD.
The only in-house option that could step-in a top-4 spot seems to be Marchenko, who actually played in the KHL against near NHL competition.
But maybe I'm getting in too deep.