Whatever role she plays, she needs to not be bitter she let Superman slip through her fingers. Is she still an engineer with John Henry Irons? That's not a bad niche for her to fill, though I kind of like the idea of her remaining in Smallville and becoming an important figure there, perhaps a mayor or sheriff or something. Someone who is relatively ordinary but actively doing something to help her community, perhaps inspired by her friend and ex-boyfriend, Clark.
Ultimately, though, I think she has to be someone who moved on and made something of herself. Engineer, mayor, what have you, she had to accomplish something. I don't think Clark would really fall for a woman who wasn't in some way a self-motivator with a bright future, even if he doesn't know it. All the women that inspire him have always been those who forged their own path, and I don't feel Lana would be an exception. Not saying she couldn't just settle down with a family and 9-to-5, but she absolutely would have moved on from Clark and not hold any ill will. I imagine they still keep in touch as friends, though very infrequently. Probably a few emails a year or some messages on WayneBook or something. She totally would have gone to the Kent-Lane wedding and been happy for both of them.
Unless we're going back to ye olde relationship drama, which please no.