Genuinely Johto is a significantly more interesting region, Johto by all accounts should be "Kanto 2" but Sinnoh feels more like that to me lol
It's funny you say this because on a side note outside of aesthetics, playing through Diamond again has me notice how scarily closely DP in particular mirrors and mimics Kanto's original game design.
The three starters (saying this even though I adore them) have the same BSTs as their Kanto equivalents at every stage, and Grotle evolves into Torterra at Level 32 while Monferno and Prinplup evolve into Infernape and Empoleon at Level 36, which is the same case with Venusaur relative to Charizard and Blastoise. The first few routes introduce Bidoof and Starly, which are perfect mirrors of Rattata and Pidgey. Starly is the first early route bird to really mimic Pidgey from a game design standpoint as a three-stage bird.
Barry's team tends to mirror Blue's throughout the game too. He starts with the starter advantageous against your own, and his first teammate is Starly, much akin to Blue's Pidgey, and he forms an FWG trio in half his team, and two constants. Blue's team was Starter/Pidgeot/Rhydon/Alakazam/two of Exeggutor/Arcanine/Gyarados depending on the starter, Barry's meanwhile is Starter/Staraptor/Heracross/Snorlax/two of Roserade/Rapidash/Floatzel depending on the starter, which is a parallel.
The level curve and boss fight arrangements are fairly similar too. Roark is a Rock-type first Gym Leader like Brock and he has a Geodude and Onix too, with his ace being Cranidos at...Level 14, like Brock's Onix, which is then followed by an overall similar level curve with Volkner capping out with a Level 50 ace, much like Giovanni in RGB, and the second Gym Leader having a Level 22 really-strong-for-that-point ace too. Even though it's at a different point there's a point where two Gym Leaders have similarly leveled Pokemon as well, in this case Maylene and Crasher Wake (in Kanto it was Koga and Sabrina). The Elite Four and Cynthia are at similar levels to the Kanto League with the same level progression, the difference being that their aces are all one level higher than their Kanto equivalents.
A lot of other similarities pop up as well. Veilstone City is a near perfect mirror of Celadon City, with a Game Corner, a Department Store, and the evil team's secret hideout, and the next major city you visit, Pastoria, follows in Fuschia City's footsteps with its own equivalent Safari Zone and like Kanto's Safari Zone, the Great Marsh has a lot of exclusive Pokemon to the Safari Game, at first just Skorupi, Croagunk, and Carnivine, but then Platinum expands these to Tangela (Tangrowth) and Yanma (Yanmega), much like how Kanto's Safari Zone had Exeggcute, Rhyhorn, and a bunch of single stage Normal-types. Platinum despite changing things up makes the parallel here even more apparent since Maylene and Wake are moved to being the 4th and 5th Gym Leaders.
And the last thing is the obvious DP dex having 151 Pokemon which is basically a mirror of Gen 1, right down to the end of the dex having the major legendaries and 151 being the Mew equivalent (Manaphy). Not to mention the minor legendary trio (Uxie, Mesprit, and Azelf) is positioned in a similar way to Articuno, Zapdos, and Moltres, the three being Level 50 encounters you meet close to the end of the game, and then in the post-game you meet several Level 70 legendaries like Mewtwo in Gen 1, although DP does it threefold with Giratina, Heatran, and Regigigas all in post-game dungeons.
I could go on and on but this is something I definitely noticed. The notion of Sinnoh being "Kanto 2" can be described as quite accurate in another regard when you look at all this, in terms of how the game is designed and structured.
