I think suicidal is fairly difficult and does not need to increase it's difficulty. Instead i prefer the developers to create some sort of AI director which could increase the gameplay difficulty by randomly changing the maps terrain, weapon spawns, trader zone and etc...
Lol AI Director. You mean procedural generation?
Anyway, I don't feel that a new difficulty is necessary although it would be nice as Suicidal does become pretty easy with all level 6 players. As others have said, I think different, more advanced specimens would be more interesting than just more health and less money.