I said it before, I'll say it again:
Roy reappearing in SSBB was an unlikely occurrence. Keep in mind that, IIRC, SSBM was released *BEFORE* FE6, so Roy was purely a promotional character with the whole "Hey look, he's going to be the new lord!" thing (similar to Luso in FFTLW, really.)
Given they claimed they are going to try to avoid clones (...which I'm skeptical on given Lucas' style seems very very similar to Ness in the previous two, though its possible they're completely rehashing Ness, and making him a totally new character in esscence), and Roy was essentially just a Marth clone. COMBINE that with how Roy is effectively a Japanese only character, something they also said they are going to try and avoid this time *punts Lucas AGAIN for that*...yeah. No, Marth isn't; given FE:DS is a remake of FE1/3, it means Marth will be localized eventually anyway, and as Elfboy noted, Nintendo stating that basically gave away that Marth returning is a no brainer. And no, Roy appearing for one scene at the end of FE7 doesn't count <.<
Given they revealed Ike a while back, a character who is not only in the US in *2* Games, but also the latest Lord in FE games, further hurts Roy's chances of getting in (they even gave Ike a Fire Attack...which is really random but that's an aside.) Really, Roy has no reason to return at this point, unless Fan Demand was high enough that they put him back in.
Why Ike and Marth? Marth was the original Lord, hence an FE Historic figure, so he has every right to represent the series. Ike the newest Lord, which is my guess for why he's in? Possible he's also popular or something, but I really don't know <_<
...ok, yeah, just had to get that off my chest!