Posts like this are going to confirm that Diluc would be the most obvious and popular choice by players. I would be willing to bet Mihoyo purposely are holding out on Diluc just to generate more hype on a later date. They could have easily dropped him as the first standard banner character to have his own promotional banner but they purposely took a 5* character with less popularity.

Using Keqing as a promo banner character is doing the bare minimum and are simply avoiding negative feedback by not releasing Qiqi as the promo character.

Could you imagine how much hate Mihoyo would get if they released Qiqi as the promo character? They basically picked the second lowest popular character just so they could generate more hype for more popular 5* characters. They're essentially holding out because that's going to make more money in the long run.

Having a Keqing promo banner is pretty concerning.