Will there be playable Firstborn at some point?

[Dexta]

?

[Sovereign]

The main guy you play as, Titus, is a Firstborn. He gets the Rubicon Primaris surgery early in the campaign but he wasn't created as one.

But if what you mean is "can we play as a marine who's never crossed the Rubicon?" then I'm afraid that's not really a thing anymore in lore. Almost every Space Marine has undergone the surgery at this point so seeing marines who haven't is pretty rare.

[Oku]

I mean technically speaking when you play as a Chaos marine in PvP you're playing as a firstborn in the most literal sense. As for pre-primaris marines, functionally no firstborns who haven't been upgraded exist.