I think CharMako makes more sense than it probably should, but I can't exactly pinpoint why...
I honestly can't see any fanfiction being made with them as a main couple, though. Unless it was really well written.
You could write one yourself. Some combi's would have never taken off if it wasn't for one person of vision promoting them.
Or, if writing a fan fic is too much, then you could request a fan fic in the writer's cafe thread. You could possibly get people interested if you can pinpoint which tropes you want the story to revolve around. Even if it is "They are both hot" trope people might be inspired to put an appropriate fic in the perv fic section.
If you can answer "why are they together" and what character types are they playing, you could get a response.