One character appearing doesn't mean they are adding those BS stages.
Yeah, but there are like 4 BSFE stages total. Whether they're there or not isn't that big of a deal (considering the lack of Voice Acting in real time thing which was pretty much the point of the BS games).
That said, I doubt that they would leave them out since they do go a long way towards showing why
Camus is a Char
Nyna has a thing for Camus.
Edit: Also confirmed for a reappearance is FE:DS1's Althena. (At this point I would wonder if some FE2 characters would also show up but technically their already represented in the form of the Pegasus Sisters plus the CC could be made to resemble either lord character or someone else as well.)