The decision was made early to not mix IPs (Mickey on top of Star Wars, etc) so I experimented with more original looks and leaned hard into the expressive faces.
Later, 3 other designers were brought on to make samples in addition to myself. I coordinated efforts and led the project over the course of the next couple months until we narrowed down the reaction sets for testing.
After a rigorous round of testing, two sets made it to the end and we went through another round of user research and internal feedback.
At this point, I was on the project solo while the other designers moved on to help with other aspects of GroupWatch. I pressed for, and was granted, one final round of iteration. I took elements from each of the other designers' reactions and combined them to create their "ultimate form." First impressions from testers indicated that the initial requested set was considered too negative, so I switched out some emotions as well.
We agreed to stick to the standard yellow color scheme for reactions. The values and features were adjusted for legibility at small sizes to pop on any background color. All the reactions were designed to fit into 1:1 squares so we could easily swap them.
Here are the final reactions in the mobile interface.