>Ruby/Sapphire/Emerald
No characters from older regions appear.
Brendan's family is from Johto
Does Bill get mentioned by Lanette?
>Fire Red/Leaf Green
No characters from older regions appear.
Bill mentions Lanette I think?
>Diamond/Pearl/Platinum
Oak appears.
>Heart Gold/Soul Silver
Never got to play them actually but I think that Steven appears to give you a Hoenn starter at some point?
>Black/White
Cynthia appears, also not sure if Wake also appears or if it's only for BW2. Looker appears I am pretty sure
>Black/White 2
Cynthia, Wake. Looker does not appear tho right? Gym leaders/Champions in PWT appear
>X/Y
Looker appears. Blue gets mentioned that he is present in the region I think? A traveler/Backpacker from Alola appears
>Omega Ruby/ Alpha Sapphire
Looker appears. AZ gets mentioned but he is not in Hoenn. Aarune is from Unova.
>Sun/Moon
Shitskin Oak, Colress, Red, Blue, other Battle Tree bosses.
Also who the fuck is Ryuki btw?
>Ultra Sun/Ultra Moon
Same as Sun and Moon but with Team Rainbow Rocket fiasco(which was executed horribly)
>Sword/Shield
Kabu is apparently from Hoenn. Giovanni is apparently there too.

Did I miss anything else?