Originally posted by Eightball:

I think the end goal in a 'no boss' run is establishing yourself in the Plains; a late stage base so to speak. At least that will be my goal for the run.

Typically I include getting the boss summon items. Not really an issue until the mountains.
But getting 5 totems makes it feel "complete" to me.

I guess I like to think of it as a world I can go back to and quickly deal with the bosses later and continue when the game expands.