Wesley was once planned to be a girl for reasons I can totally relate to (How many promising sons are there in fiction?). Unfortunately Gene said no because dang it, Wesley ain't a girl's name!
I think it was Bob Justman who lobbied for Wesley to be a girl, because he felt that television hadn't really done much to explore "mom and daughter" stories, and that it'd make for an interesting dynamic change. The early casting sheet for the series, as well as at least one story outline of "Encounter At Farpoint" by D.C. Fontana, made the assumption of the cast containing a 'Lesley Crusher'. But as we all know, Wesley as a boy won out.
I agree, it's a shame. "Smart whizz-kid boy" is such a cliched trope in fiction it would have been far more interesting to have seen TNG do it with a girl instead.