There are no plans to introduce modes of play to accommodate 5 or 6 players. Grant, the designer of Imperius, laid out his reasoning recently on BGG: https://boardgamegeek.com/thread/1964864/why-4-players
"More than four would be absolute chaos. Your decisions wouldn’t matter.
A common thread in my designs is managing uncertainty and heavy player interaction. I’m not able to go beyond four players without compromising those.
I guess we could make a fifth and six player house, and I’ve tested it before, but it doesn’t work, and I wouldn’t want to put my name on it. Imperius is 2-4 players, and it is great at 2-4 players." - Grant Rodiek