Between TheLostWhisperer and I analyzing the trailer over and over, we realized that right at the end when Byleth crits her voice is different from the narrator's. We've already confirmed Christina Vee is the narrator who is Edelgard/Edgelord's voice and the fact that she specified who she played (in a Twitter post) implies somebody else voices Byleth. Not to mention her voice only came in at the exact moment she critted, so this could justify her being female despite the general assumption that it's a male. Therefore this could mean they, regardless of gender, could be playable besides another stand in role and given how androgynous he/she appears to look (could tie to demonic backstory given name) it seems fitting for the avatar.