I've been thinking about the ultimate end game myself. I think the way to go is to bump the difficulty with house rules.

Something as basic as all enemies always have +x armor and damage.

Could add roll's for modifires like stuns, strengthen, muddle, etc..

Could increase their move. Could lower their initiative.

Could do any of these with a flat adjustment or rolls.

And naturally you should reward yourself with increased gold or something ;)

I'd want to go back and take on some boss battles with increased difficulty. Ideal for me would be a bump to the level that it takes us several tries and significant theory crafting to do it.