Does anyone know who is going to be apart of this legendary banner?

Yes.

https://fireemblem.gamepress.gg/q-a/legendary-banner-prediction-0 Self advertisement intensifies.

Legendaries/Mythics confirmed: Fjorm, Marth, Hector, Eir

Akariss predictions:
Red: flying Tiki, Flora
Blue: tome Camilla, Quan
Green: SF Nino, B.Ephraim or new legendary
Colourless: S.Takumi, Loki or new legendary

My predictions:
Red: flying Tiki very likely. Flora is a good guess; if not her I would look for Laegjarn, B.Celica, or possibly even a new legendary.
Blue: SS Camilla very likely; possibly new legendary because Fjorm has received so many runs.
Green: SF Nino very likely; B.Ephraim is a good guess and I'm not sure who else they can put here besides a new legendary. Hector has received two runs so far so it's not too big of a stretch to see him with a new legendary.
Colourless: either S.Linde or S.Takumi, and I'm betting on Takumi because S.Linde has really similar stats to Eir. Loki seems pretty reasonable as a guess; I originally thought B.Veronica. Akariss says he doesn't feel like it's her time to return yet, and if B.Ephraim or B.Celica are coming then I can see Loki being chosen over Veronica.

Eir has yet to appear in the legendary rotation proper. So, I hate to say this, but it won't be a brand new legendary; instead, the spotlight will be Eir's.

As for others, I'm praying that the seasonals are all Sketchy Summer, so I can skip and focus on New Year's, then beasts.