Note that Captain Falcon and Ness shared an exclusive quality - they were both the sole and main representatives of their franchises. Chances are that Sakurai noticed this and decided to make them starters in Melee because, after all, it'd make sense for star characters to be delegated to star positions (i.e. starting positions). Marth and Roy were virtually unknown outside of Japan, so they hadn't really reached star status yet and Mr. Game & Watch had a special role as the surprise unlockable. Since Marth is widely regarded to be the face of Fire Emblem and Fire Emblem has finally been brought overseas, I think it's likely that he'll become a starter for the reason I stated above - same for Mr. Game & Watch (who also will have lost his surprise aspect, which is just another reason to make him a starter).