Only just had a chance to even look at this post and I think it's a great mind poker. :\
What about say - a custom Classics Shockwave that has his arms from a Classics Prime on the base of an Energon Shockwave?
Things like that which is kind of a frankenstein custom... So - not custom built or scratchbuilt per se -

Or a Classics Jazz that is really a prowl - but with say - a robotheroes jazz head- (i've heard of someone doing that )