Not much unless you're looking to make a character extremely specific with certain things, like G.U.N. and such.

Obviously with the core changing event allowing the Echidna to live into the modern day, their species is on the table though they aren't a very common sight off of Angel Island aside from Knuckles and, in this universe he gains a little sister with his family named Onyx.

Thinking on it a bit though, the early events did differ pretty widely so I may add a few examples in a bit-