I think that the Saints might just get corporatised and restrain itself from its gang image(as they are pretty boss after i completed the game), which over the years diffrent gangs take over unoccupied saints areas like Stillwater and then evently steelport(considering that the saints loses their grip on stillwater because the conscern was to take over steeleport and they lost all their revenue gained). I think that its possible that you choose/ create a new main character that is involved with a new chapter of the 3rd St Saints, that try and reclaim its gangs former glory.
But i assume the next game will eventually go back to stillwater to reclaim it from new gangs.
or the saints get hit by major crime organisations like Triad, Mafia, and or military based U.N.
you expect the game to take the saints to a global level, but thinking in retrospec that if it went in that direction the game may have to loose its sandbox feel as it may be too big, though they may only just have to take over 4 cities(one in each region).
It makes me sad that the Saints are all corporate even when they were broke during the start of SS:TT they still felt corporate(still fun though).