Imo we will see other Lunarians. Oda hasn’t really too deep into lunarians, we only know king so far. At the end of series, we might be seeing the lunarians if the red line were to be destroyed.

We will know the seraphim

They were said to live "on top" of the red line, I don't see how the red line being destroyed will make them appear

We technically already are

No genuine Lunarians I think

If you count the seraphim then we already have but still

Maybe in a backstory of some sort but currently though, I honestly don't think so.