I think a Jack/Elizabeth relationship would be VERY interesting, but I don't think it'd ever happen. You know Disney films -- they almost always have romance, and it's pretty clear Will & Elizabeth are "The Couple", while Jack is "The Star". They don't tend to mess around with that kind of formula.
Besides, if they didn't do it right, it would turn into a soap too easily. So it's probably best they don't do it... even if I think it would be kinda interesting... 😮 Oh well.