Honestly, they shouldn't bring back Odyssey yet.
Don't get me wrong, I absolutely loved that game mode. Getting all the missions done was probably the most rewarding and satisfying experience I had in all the time I am playing this game. But bringing back Odyssey when they just added 1 new skin to it (which would have to most likely end up as a boss alternative for Kayn) would feel lackluster.
They should create a new line of skins for Odyssey, a completely new crew with their own unique augments for this game mode. Just playing this game mode with the same 5 champions we played it the first time wouldn't be as exciting, since there wouldn't be alot of new stuff to discover. To really hype this game mode up again, they should go the full way and do another event with new skins.
Some examples for possible new skins:
Some kind of ora priest, filling Sona's role.
Engineer of the new crew, filling Ziggs' role.
Ekko's battle robot, fueled with ora, filling Malphites role.
Former headhunter, filling Jinx' role.
Captain of the new crew, filling Yasuo's role.