I actually doubt that solo-mode will become a thing, unless bots can suddenly handle events easily, which would make them perhaps broken.
We´ve private lobbies for 2 and they´re clearly needed for certain events, especially those with the nurgle-eye on heresy / damnation. The tentacles go way too fast back on spot for just one person. Might be possible at some spots or with well-placed grenades, but definately not at all…