Personally, I don't like the idea of splitting by brand or exotic/ mainstream. Sure, there are lots of brand specific questions, but is brand specific tech really the reason we come to ADV rider? Personally, I go to KTM talk or Thumpertalk for that. IMO, following the same basic format of those pages will diminish the uniquness of ADVrider...which seems to be it's focus on how all brands compare and can be applied to various adventures.
A repeating theme I have noticed on Thumpers is that there are several schools of thought on what constitutes an "Adventure", and they usually steer the bike criteria and the discussion. For example, is it a 90/10 Thumper, 50/50, or 80/20 (off road/ road)? My suggestion would be to split the forum into those three categories. That way, we still get to contrast the various brands against one another, and any bike may show up in any forum. But, the sub-forum will automatically communicates our intended use for the bike and steer the feedback/ conversation.