Israel needs to be proactive in this. If Hamas thinks hostages, dead or alive, are worth x number of Hamas prisoners, and soldiers worth y, plus humanitarian aid and cessation of hostilities and reconstruction of Gaza, Israel should take that calculation and run with it, even with some adjustments. Translate it into some equivalent calculation and start executing, ie give the death penalty which is on Israeli law, to captured Hamas terrorists. Try them in military court, sentence them (my assumption is that this should be a fairly easy and straightforward trial - there is not much of a defense for what they did), and start publicly executing them according to the calculations of the hostage deal "accepted" by Hamas. If Hamas gets upset, they can trade hostages to save their terrorists from the firing squad.

I would focus, or at least start, with members of the Nukhba force that have been captured, and if necessary move on to others. 

With the assumption/knowledge being that Hamas has been killing, or leaving to die, the Israeli hostages, and likely most are dead by now, they need to pay for that. It would be somewhat revenge but also perhaps, hopefully, a deterrent to get Hamas to keep the rest alive for as long as possible and to stop dilly dallying and to get moving on a realistic deal before too many of theirs are dead.