For extra characters, I think they should handle as many as possible as being same characater, different "costume". If Nintendo goes for as many characters as possible, many of them will be slightly altered clones and will seem ridiculous.
Marth & Roy are great, but I don't feel like also having Eliwood and Ephraim's sister added to the mix. SSB has a higher tolerance for more characters then other games simply because of what it represents, but that tolerance does not allow too much more flexibility (and the general rule is each character can have one near clone).