Well you post didn't specify so I could only assume.I do not have the Wii version, but coop play is currentlynnot supported in the playset worlds on the Wii version.
Supposedly, they are working on a fix.
It has always been very well known that only "franchise" characters would be allowed in their worlds. That has nothing to do with it.
As per this it looks like the reason was essentially because of the Wii's limitations on the size disk it supports so I'm not sure how they could be working on a fix but hey if they are, great!