I never grew up on Johto so I don't have some deep connection to it that blinds my view, at the same time it isn't one of my favorite regions either, but what it manages to do and how it incorporates 2 regions into the game is quite remarkable. It's not a simple rehashing of the same old experience, instead, the battles are much harder, the way you traverse the region is different, and areas like Viridian forest, that originally had ominous music indicating a sprawling, arduous adventure, are upbeat and give off a tone that resembles what you would expect for a seasoned trainer like yourself.
Johto utilized architecture and age as motifs for world building, where you explore ruins, see legends in traditional elaborate buildings and the ruins of old buildings, not to mention a feeling of rebirth of new legendaries coming out of the ashes, and becoming stronger through a dragon challenge set in the dank caves. It really pushed the limits of what it's predecessor had done, and not only had a coherent and unique style to set it apart as its own thing, it incorporated a newer modified version of the old region you had spent so much time in, and did it in such a way as to give the impression that you're coming back to a familiar place, but it has changed as you have changed.
It even included one of my favorite features in the remake at least where your pokemon followed you around, which honestly helped to set the idea that you were travelling with your pokemon as partners rather than a master ordering around tools to do your bidding
It's really interesting that Nintendo had never done anything quite like Johto, where you could revisit a past region, it was only unique to Johto, the only thing close is black 2 and white 2 where its the same region but new areas to explore, though that was a direct sequel and not a new generation. Sun and moon had made mention of Kanto, but refuses to deliver leaving everyone high and dry.