So i know it is late but i feel Gaius will have some role in EW even though we haven't seen much of him in the promotional. First off the big one we are going to Garlemald, While Gaius has gone back on his might makes right philosophy he has still stated his desire to protect Garlemalds citizens in the Dialogue in 4.5 and seems to want to work towards its reforming.

Second of all it is speculated that one of the red masks Gaius had was from Fandaniel. This was shown in patch 4.3 right after Asahi was killed so it would line up perfectly.

Finally the Raid series will deal with Lahahbrea in some form or another, whether it is him or something he left behind has yet to be seen but either way Gaius stated in 4.5 that he was hoping Lahabrea would be his final opponent so it would tie back to that. These point line up so much that him not being there would feel really weird. Yeah you could argue Werlyt was his sendoff but EW has far more characters, locations, and plot points that I would argue are important to his character's history.