As sweet as Mutsumi is and as great a match for Keitaro as she is, it'll never work.
Keitaro met her too late and was already completely in love with Naru at the time. He never seriously considered Mutsumi as a potential lover. He had complete tunnel vision, even when she basically threw herself at him, something that 6 months earlier would have quickly ended in a roll in the hay and a proposal, he is at best indecisive.
If Naru dumped Keitaro, or better yet behaved in such a way as to make him dump her (knocking boots with someone else, attacking his british friend for no reason while screaming that Keitaro is hers and he could never have him, etc), Kei might, I stress might, in time approach Mutsumi for a relationship. This would be after several months as Kei is a fairly sensitive fellow and it would take him quite a while to be able to trust again. He wouldn't go to her for a direct rebound as it's not in his character. He doesn't use people like that. But even then it would be rocky. He would be settling for number 2 in his heart, and she would know that. In time it would drive them apart unless they took great steps to cement their relationship, effort that most people never make. Perhaps they could learn to trust each other, Mutsumi learn to value herself a bit more, and Keitaro learn to value her as well. Human nature being what it is, we never value the things that come easily, only the things that we have to work for. For it to last they would have to work for their love.
So yeah, that's the romance I can see coming from a Keitaro/Mutsumi pairing. Serious, emotional and deep. It'd be a bitch to write well. You would need to pay constant attention to tone and be very careful with the supporting cast. If you tried to keep to the usual frenetic pace of Love Hina I just don't see it working. Naru's ridiculous indecisiveness and insecurity was a vital element of dramatic and romantic tension. Remove it, and the series winds down rather quickly unless you shift gears and tell a more human, emotional story. I'm not saying there would be no comedy involved, but it would be much more subdued than your average LH story.
and as always, that's just my $.02
-exar out