How about starting with a couple of 'catch-all' forums then see if you have to add more. egigs already has a lot of boards going on, adding a lot more may just splinter things too much.
Why not call them;
'Plectrum-pickers - A forum for anything music related that doesn't fit into the other categories. If in doubt, please post here'.
' Bands & Artists -If you want to discuss bands or musicians, new albums, future releases, favorite songs or 'Top 10 guitarists of all time'... Please do it in here to keep the rest of the forums as focussed as possible'.