It should be noted, again, that Marth's is more of a FE3 Battle medley as it has the first part from Book 1 (And Shadow Dragon, but /not/ FE1) and the second part is Book 2's (And New Mystery of the Emblem's).
As for Roy's theme, he doesn't really have one akin to The Devoted or Id [Hope]. The normal boss music is bleh, the regular battle music is bleh...
That leaves the unique boss themes. Shaman in the Dark is a no go for obvious reasons, as is Battle for Whose Sake (Zephiel's battle theme).
That only leaves In The Name Of Bern. It would be awesome, but it's...not fitting for him at all. And the arena battle music is just Genealogy's which would be better saved for a potential Sigurd or Seliph fight.