A potential problem for a pre SSX wife for Liu Bei is either Liu Bei and SSX are flirting while he is married or she suddenly marries him soon after he became a widower having had no flirting before. Whereas right now they have both get time to build a romance (and then she vanishes off the face of the earth once they marry)... Having said that, acknowledging 3kingdoms China wasn't full of one true love (bar Xun Can and Lady Cao!) wouldn't hurt.
I'm pro Sun Luban, would prefer Gan over Mi (whose appeal I have never managed to understand)