I think I've narrowed it down to an SG or LP Special shape (double-cut, flat top). I'm actually kind of leaning more toward the LPS shape because it is more "generic", if that makes sense - I dunno, I think it would feel odd building a guitar with a shape as distinctive as an SG, but without it actually being an SG (meaning, all the necks are 25-1/2 in scale, bolt-on - which makes it really not an SG, ya know?) I just think if I were to build something that looks like an SG, it should be a 24-3/4 in scale, set neck....but maybe I'm overthinking it too much, or being a little to OCD! Same reason I am not really looking at the LP shape. Now, I know a "real" LP Special is 24-3/4 in scale, set neck also, but I think that double-cut flat top shape is not quite as "proprietary" as the LP or SG shapes, so I feel like it being a 25-1/2 scale bolt neck would be "acceptable". (I think I'm not making any sense...
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I'm also not sure about the wood type - kind of thinking alder or mahogany, but then the Ric 12 strings are made out of maple.....I just think maple would be too bright.
Then the pickups - P90s or Gretsch-style TV Jones filtertrons? Or just plain old vintage 'buckers (I don't think you would want anything too hot on a 12 string!)
Decisions, decisions.....