I'm someone who believes people just don't die in One Piece. Call me naive, stupid, or stubborn, I just don't believe characters who are named die. I'm not going to argue this, rather just wanted to state this.
As for who comes back…
Eneru: Not likely. That dude has found his utopia (or assume he has, we don't really know yet). Why bother going back to boring old inferior Blue Seas?
Kuro: He could, but he would suck. Honestly, Kuro makes a great East Blue villian, but would suck on the Grand Line. He lacks the power to take on Grand Line foes, and careful planning is somewhat useless in a place were unpredictable pirates rule. As cool as Kuro was, he's not a Grand Line dude.
Arlong: On the assumption he's alive, which could be wrong, I could see him coming back. Especially once the crew reaches the merman/fishman (I'm still confused exactly which species lives there or if it's both) island.
Krieg: I'd like him and Gin to come back. If they will is kinda up in the air. I think Oda himself didn't know. He made it sound like Gin was going to meet Sanji later, but at another point Gin says stuff that makes it sound like he's short on time. Maybe Gin is alive, perhaps he's dead or too sick to fight. Either way, Krieg is a toss up.
Croc: I have no idea. The whole Impel Down thing could be a way to end the story of the Baroque Works group, or it could be an elaborate set up for the next meeting of the group. I'd want to say Mr. 2 should make a 2nd appearance, but I see no reason for Mr. 1 or 3 to visit the group.
Mr. 1: So uh Zoro... still remember how to cut steel?
Zoro: Yea, I'm cool.
Mr. 1: Alright... sigh I'll be over there then...