Well I don't know all of them, but Unova is mentioned on the Pokemon sport show thing on tv (can't remember the real name) and I believe there's a house in snowbelle city that plays the snowpoint city theme from sinnoh. There's also a guy in a random hotel that says he comes from another region that isn't kanto,johto,hoenn,sinnoh or unova.
That's all I can think of right now
I can't believe I forgot the snorlax on the bridge on route 7, the mewtwo in the unknown dungeon in Pokemon village, the roaming legendary birds, and how santalune forest is like viridian forest like how you find pikachu rarely in both of the forest
These are all kanto references
Can't believe I forgot about professor sycamore giving you the Kanto starters
Another kanto reference
Hope I helped