There are a lot of lessons to be learned through the development process and final product, but I also think it's useful to remember the context of SMWCP2's development. SMWCP2 started in many ways as a "reaction" to the faults of SMWCP, particularly the prevalence of item babysitting, inconsistent difficulty, uninspired level/world theming, etc. There was a lot of joy in the early parts of SMWCP2's development, particularly related to the sheer creativity and more consistent design approach. I particularly remember the fun of helping brainstorm world themes, and moving away from the grasslands-desert-ice-forest-etc etc format. Arguably, SMWCP2 could have been much more economical in its use of creative assets (if space limitations are anything to go by), but I think the imagination behind the level design of this project still stands as a highlight.
SMWCP3 could definitely benefit from SMWCP2's lessons, particularly in design economy and a planned development cycle, but it also shouldn't be afraid to get weird and optimize the creative ambitious that this project couldn't fully hit.