Satella might be Subaru daughter in the future. I mean, her name Satella = Subaru + Emilia
I don't think too overly complicated for this. I remembered in Rave by Hiro Mashima, the FMC which is ELIE is a historical figure who get sent into the future and forgot all her memory but the number of 3173 in her arm, which is being read as ELIE. It's fun but impactful. So,
SA+ELA = SubAru + EmiLiA
Satella might be Subaru daughter in the future. I mean, her name Satella = Subaru + Emilia