no idea, and since my parents split when i was really little, i don't think i can use their other kids' names as an indicator to what kind of name it might have been. they went in pretty different directions after me... though it is kind of amusing to speculate whether i'd have been the "little john" instead of my (half) brother, who was named after my dad because he was his first son (as far as he knew, anyway).
the only time i ever asked my parents anything like that was to ask if they had actually wanted a girl, which of course they assured me they had. no one is going to tell their five year old they wished they were the opposite gender. but being a small child, i took their word for it and thus the question of what might have been was never brought up. if they wanted a girl, then as far as i was concerned, that meant there was no unused boy name floating around anyway.
i'd be tempted to ask my dad now, though i'm not sure how i'd word the question. he doesn't really understand the whole gender thing and still seems to be under the impression that i'm just a lesbian. i'd never ask this to my mom, though. she'd probably throw a fit at the mere mention of it.