You have to have I think about 50 percent of the museum donated, all categories included, and then blathers will ask you to make it a public works project.
You first have to donate at least 20 items to the museum. You also have to donate at least one item to each of the four categories: bugs, fish/deep sea creatures, fossils, works of art. After this, you will one day find Blathers thinking about something when you first go into the museum.
It sounds like you have everything you need. Just gotta wait now. The whole randomness of that part is annoying. Took me two weeks after I had hit my prerequisites for him to be thinking about it. Good luck!
Have at least 20 items, an item in each category. Speak to Blathers everyday for a week or two (for me it was a week) and when you come in he'll be thinking about something.