Now that C9 has likely solidified their roster, here's some ideas for what we can expect from them on each map. With there being some overlap between Zellsis and leaf, as well as the question of whether or not yay will start playing Jett more if the meta starts to shift a little back towards Jett + Killjoy/Cypher, this is completely speculatory and could be completely wrong.
I've heard that leaf is actually an extremely flexible player and might be somewhat transitioning to controller/sentinel on some maps, so that will reflect here,
Ascent:
yay - Chamber
Zellsis - KAY/O
leaf - Neon / Raze
Xeppaa - Fade
vanity - Astra / Omen
Alternative comp:
yay - Jett
Zellsis - KAY/O
leaf - Killjoy
Xeppaa - Fade / Sova
vanity - Omen
Bind:
yay - Chamber
Zellsis - Raze
leaf - Viper
Xeppaa - Fade / Skye
vanity - Brimstone / Astra
Haven:
hear me out here, I couldn't really think of how to make a standard comp work so I decided to bring out the OpTic comp
yay - Chamber
Zellsis - Phoenix
leaf - Killjoy
Xeppaa - Fade
vanity - Omen
Icebox:
yay - Chamber
Zellsis - KAY/O
leaf - Sage / Raze
Xeppaa - Fade
vanity - Viper
Alternative comp:
yay - Chamber
Zellsis - Sage
leaf - Killjoy
Xeppaa - Sova
vanity - Viper
Breeze:
yay - Chamber
Zellsis - KAY/O
leaf - Neon / Jett
Xeppaa - Sova
vanity - Viper
Alternative comp:
yay - Jett
Zellsis - KAY/O
leaf - Cypher / Chamber
Xeppaa - Sova
vanity - Viper
Fracture:
yay - Chamber
Zellsis - KAY/O / Raze
leaf - Neon
Xeppaa - Breach
vanity - Brimstone
Pearl:
yay - Chamber
Zellsis - KAY/O
leaf - Viper
Xeppaa - Fade
vanity - Astra