Assuming the dlc routes follow generally the same structure as the four base game routes, I tried to find the first recruit in the dlc routes. My choices are based on what weapon types the teams already have access to, what niche the teams lack and what neutral characters are left. With neutral characters, I mean any characters that aren´t already participants in the Heroic Games, as well as characters that are already the first recruit in any other route.
Fiery Mane
Betrand-Axes
Gaitz- Lances and Mounted
Dante- Black Magic
Goliath- Axes and Armored
Jester- Swords
I think Peppe is the most likely choice, since Bertrand's team lacks a bow user, and you need to do Bertrand's paralogue to recruit Peppe in the other routes. However, his team also lacks a dedicated healer. Something every single other team has access to, but there aren't any neutral healers left. Dietrich gets Yang Jie, and Theo gets Sophia. I guess they could also do a repeat with the first recruit.
Tale of the Moon
Talimun- Black Magic
Ursula- Bows
Simon- Axes
Ludia- Swords
Fiana- White Magic
What Talimun's team lacks is a lance user. There are two neutral units left (Leda gets Catania), who could fulfil this role are: Nydine and Io. Both of them are mounted as well, so a perfect fit.
Bridal Party of the Pale Raven
Anatolia- Gauntlets and Black Magic
Shan Lan- Gauntlets and White Magic
Nezha- Swords
Dadao(Ugh)- Axes
Halvin- Bows
Again a lance user is missing. So Nydine or Io.
Linaria Dewdrops
Orchel- White Magic and Axes
Diegos- Bows
Seteh- Lances and Mounted
Ninae- Lances and Mounted
Loretta- Swords
The fact that Seteth and Ninae fulfil very similar roles is very strange. A dedicated Black Magic user would be best here, but there aren´t any neutral units like this at all. Another thing this team lacks is a Gauntlet user, so I think Noctula could be the first recruit. She is easy to recruit but isn´t the first one in any route.