>>45727052a few from my most recent HG/SS play through:
you can get the OG Kanto fossils from the rocks at the Ruins of Alph. Can't do anything with them, but they're there
most people don't know about the Lance/Clair double battle against you and your rival, he just hints he'll go train in Dragon's Den like the originals so you have to go check on him to find that
Misdreavous wasn't available until Mt Silver in the originals, but you can get one pretty early in Cliff Cave in the remakes.
Similarly, you can get Murkrow and Larvitar post-Amphy at the Safari Zone (although it's a pain).
Most people know about the Slugma in Violet City by now I think
