A Bug type legendary is the only one I can see possibly making sense. I mean, we have Celebi who is the "guardian of the forest", so a Bug type could fill a similar role.
A Fighting type would just seem to restricted... What would he be the legendary of? Unless he had some ancient, intense battle with Arceus over dominion of the universe and thus was the first "fighting" type, it just wouldn't make sense to me.
Though a Poison type, man-made as suggested by timbjerr, would potentially work. I can't see it as being one of the top-tier legendaries, but possibly somewhere above the pseudo-legendaries (Dragonite, etc.) and below Mewtwo and Celebi (unspecialized legendaries, etc.)