Newer Pokemon that should appear in older regions
I'll start with a few:

Kanto:
Starly line (Pewter City)
Tyranitar line, Hydreigon line, Druddigon (Victory Road, possibly also Safari Zone)
Zebstrika line

Johto:
The obvious. Murkrow, Houndour, Slugma, ect
Flechling line (Ekruteak City)
Chimecho line (Ekruteak City, Mt Mortar)
Throh, Sawk (Mt Mortar, maybe some other caves/outside areas)

Hoenn:
Should have more Fire-Type/Grass-Type Pokemon, given it's tropical nature
This could include Arcanine line, Exeggutor line, Chandelure line, Salazzle line, ect

Sinnoh:
Also needs more Fire-Types
The Chandelure line could be found in the Old Chateau. Some others could include Darmanitan, Pyroar, Talonflame, ect

Unova:
Many Alolan Pokemon could be found in Unova, since they're in the same country

Kalos:
Alcremie line

Alola:
Alola could have it's own Safari Zone dedicated to Kantonian/Non-Alolan forms of Pokemon

Galar:
All the Pokemon taken out of the game