Jack will probably have a pretty mellow buddy relationship with Mal after all these years, and be there to hand him the whisky when he needs it, whether he goes down the drunk route or not. :-)
Martha - Jack is definitely going to have periods when he's sleeping around outside the Ianto/Spike relationship, so if Martha can't get herself a partner, Jack's very happy to have had an off and on again relationship with her. Also very happy to wake up beside her if she doesn't get any better offers. :-)