This season, we've really seen Kevin become more mature through his relationship with Zoe. If that relationship continues to grow, we wouldn't be surprised if the couple is married in the flash-forward. This season has hinted at the possibility of Kevin settling down and starting a family. There's also a theory that if Kate dies, Kevin would actually help Toby raise the baby as his own.
Of course, this is This Is Us, and they love to throw us curveballs. It would be just like the show to have Kevin pop up with someone completely new when he visits Rebecca. Or perhaps he ends up getting back together with Sophie in the end. Anything could happen!