All they would really need to do is wind up Will Ferrell, John C. Reilly, Adam Scott, and all of the rest of the cast, and it would take care of itself.
The original Step Brothers felt built mainly on improv between its main ensemble of actors, and it was not a particularly narrative-driven film.
If Adam McKay wanted to get the gang back together for another movie, there would be a lot of flexibility and room for comedy.