Not sure if anyone already adressed the topic (if they did too bad I’ll talk about it anyway), but I have a strong believe it Will be M!Byleth. Why? Its Simple:
1- all the 3H lords Will received a B! Alt this month
2- It comes around the same time the game makes its 1 year anniversary
3- All colors are Open this month
4- F! Byleth got an Alt so I doubt they would give her another alt so soon (unless they want to make her Camilla 2.0).
Anyway, if had to bet on anyone else it would probably be either Corrin or Ninian and if they want to give us a curveball: Sigurd or Gustav.
But its I.S, so anything goes for them