(http://i.imgur.com/bkcPm.png)
Something I mocked up after I wrote my post about SO2 rehaul, I think it'd go down pretty nice, i might try to !@#$ with a real image later on, for now, !@#$ it. I'll leave it at this.
As for the "cave", this was actually inspired by Cave Story when at the end of the game you can do the "HARDCORE" mission and kill that guy and save the island, going down the cave and fighting some hard ass mother!@#$ers would inspire teamwork, etc. anyway, i'm out for the night, i'll talk moar about this later.
While that is good and dandy, the current game engine wouldn't function along well with any of these ideas. For one, if you haven't noticed, the world is seamless. Meaning it is only one individual room, there are no doors to new areas. Second, there is not scenery that interactions with players (ie steam blasts killing players), so that wouldn't register well.
I approve your idea in one point. Regarding the amount of time used to travel through the whole world, it should grow in a vertical way instead of new content added to the right side every time.