Salomaki, Hartman, Emelin, Hartnell, Fisher, and Saros need new contracts now (if Poile wants to re-sign them all - I think at least Hartnell and Fisher probably don't come back, and Emelin may not either). I don't think any of them are going to be terribly expensive. You may also see extensions for Fiala, Sissons, Ellis, Bitetto, and Rinne signed this summer since they're entering the last season of their current deals.
Rinne will probably be signing a short contract worth much less than the $7m/season he's making now. Maybe that will cover Ellis' raise. Then the question mark is how much Fiala gets, but I think he'll probably be a bridge deal around $3.5-4m/season.
My point is, I'm not sure it makes much sense to trade Subban or any of the top 4 D if you're just looking for cap space. There should be enough cap space already to pick up someone in the $4-5m range to improve the second line scoring punch. Maybe even quite a bit more, depending on exactly what the cap ceiling does.