I don't like Jack/Gwen.
Because Jack is invested in her having a life outside of Torchwood. Jack is, in many ways, Torchwood. Thus, I just don't see it happening. But this is Torchwood.
Unless Gwen breaks up with Rhys in the first episode. Jack then tries to fix her up with some "nice, normal people" - which is really bound to include at least one alien, but the one that tries to kill her is the accountant - and they have guilty UST for a few more episodes before Jack finally gives in around episode nine.
Of coures, they would destruct in the final episodes because Jack can't keep it in his pants any better than Gwen and she can't handle being cheated on. And, maybe, she finds a nice guy at the end.