Not to bump my own thread, but here's another good one from Philly. It's the Joint Library. A massive Egyptian Revival building on Washington Square. It was going to house several organizations, most notably the Athenaum of Philadelphia and the Academy of Natural Sciences.
Here's a thought: what if the Academy of Natural Sciences, the existing museum, eventually grew into this building. I don't know if the others would have lasted for 200 years in the same space, but the AoNS kept on growing. Imagine if that building were the Natural History Museum now. I could see it. Plus, it's right by Independence Hall and other tourist favorites.
Image:
https://www.philadelphiabuildings.or.../0/default.jpg