To me I'd personally say that Gen 3 was sort of the experimental generation that Gen 4 built upon and made its identity with its games. Gen 4 is praised for a lot of reasons, aside from the fact that it was the era when the second wave of Pokemon fans rolled in, but in many ways the big things it has going for it (and this applies to the DS era as a whole) aside from that green_typhlosion mentioned is the sheer amount of fun content the games have. The games are still pretty simple in terms of story, but what they have going for them is content. The regions are great to explore, but what DPP and HGSS offer is the amount of stuff you can do after becoming Champion. DP had a whole lot to explore after defeating Cynthia in Route 224 and the Battle Zone, and it brought back Contests from Gen 3. Platinum took it even further and added a Battle Frontier, which returned in HGSS. HGSS brought back Kanto but expanded upon it so there was genuinely more to do there, and added loads of content that made for a game that gave you so, so much to do aside from the main campaign: it modernized GSC by adding more stuff like Pokeathlon, the Frontier, more legendaries to find in Kanto, and vice versa plus stuff like Gym Leader rematches, the Safari Zone that is customizable, and vice versa.
The DS era was all about having more to explore and more to do in those new areas. Gen 5 had this too, with BW1 having half the region left unexplored for post-game, more post-game dungeons, and Alder+bonus bosses in Cynthia and Morimoto, and BW2 did this too with more of the region to explore, Pokemon World Tournament, and more.
Each game in Gen 3 was sort of the experimental stepping stone that led to the creation of what Gen 4 was all about and what defined the DS era the most. Ruby and Sapphire had a pretty barebones postgame like the two gens before it, but it added Contests and Secret Bases as fun features that DPP retained. FRLG added the Sevii Islands as new areas to explore and train your Pokemon, which was a big aspect of Gen 4/5 postgames in DPP, HGSS, BW, and BW2 with the Battle Zone, Kanto, and certain parts of Unova. Sevii was sort of the building block that led to all that. Emerald had the Battle Frontier as a full blown multi-battle facility post-game to do, which Platinum and HGSS brought back in their own spin.
Of course, that's not to say that Gen 3 was the only experimental/testing ground generation for the gen/era that came after. Gen 5, despite sticking to many things Gen 4 did, was also an experimental ground in and of itself. And how? It tried to experiment with adding a story to tell in Pokemon games. One of the reasons Gen 5 gets praise now is it had a really cool story to a Pokemon game to make it feel even more like an RPG, and this is something that would define the 3DS era. The 3DS era, with Gen 6 and especially Gen 7, was all about trying to create a story alongside the adventure beyond the simple "Get 8 badges, fill that Dex, and become the Champion", and tried to create characters and a story arc for many of them to make a more story-driven experience. This is aside from the fact that Gen 6 brought in yet *another* wave of Pokemon fans with new kids joining the Pokemon craze (aka the third wave), and in many ways like Gen 4 it also brought a lot of modernizations and innovations.
You can see this in how the 3DS games have a lot of story-driven elements in their adventures. XY's execution was a bit of a mess, but it definitely tried to have a story to tell, especially when one of its post-game quests is also a story in and of itself with Looker and Emma. ORAS also focused a lot on story, by fleshing out the characters and turning the barebones story of RSE into something that was genuinely quite cool to experience. Not to mention the post-game Delta Episode that added more to the lore of Hoenn. SM and USUM are, of course, incredibly story driven, especially the former. The games like SM and USUM and even the likes of ORAS have a lot of this, and even now there are people who *do* praise them because they loved the story they experiences with these games. USUM added quite a bit in terms of story and lore by adding a post-game story to call its own as well, with the Rainbow Rocket ep and looking at the Lillie/Gladion/Lusamine dynamic in a different angle from SM. Some could say the overall execution was worse (and in some ways, it is), but it's also an alternate angle at some of the characters' dynamics and personalities that SM didn't touch upon.
(I unfortunately can't really pinpoint on what Gen 7 tried to experiment with that Gen 8 expanded upon, because rn we only have Sword and Shield so I can't tell what the Switch era's schtick is, but if anyone can identify it I'd love to have some input).
A bit of a side note from that, but as an unpopular opinion in and of itself. I think Omega Ruby and Alpha Sapphire are really great remakes, and overall really great games. I think a lot of people were a bit disappointed in them because they compared it to HeartGold and SoulSilver in terms of how they fleshed out and expanded upon their source games, but I think ORAS is also an awesome game and one of my favorites in the series, but I think they improve upon the original RSE differently. As such, I also really like ORAS for different reasons than HGSS. While HGSS fleshed out its source game by adding loads of awesome content to do in the post-game, and having a lot of stuff to do and more places to explore, I think where ORAS excels instead is in taking the Gen 5 approach and fleshing out the story. It took a rather weak and barebones story from Ruby and Sapphire and made it genuinely interesting to experience, by expanding upon the lore of Hoenn, fleshing out the characters to make them more interesting, and creating a truly enjoyable adventure to experience in Hoenn. That's not even to mention how it made Hoenn even more beautiful and seeing it in a modern light was so cool, just as HGSS did in making GSC Johto even better and more awesome. Plus while it didn't quite add the scope of content that HGSS had, it did still have a lot of cool features despite not having a Frontier. Soaring across Hoenn on Latios/Latias was awesome, and getting an overhead view of Hoenn was and still is cool, and finding a lot of Mirage Spots and tons of legendaries was still awesome, plus upgraded E4/Steven rematches like HGSS did with its own E4 and Lance. Not to mention the return of Contests, which are even better than in Gen 3. Overall I do really like ORAS, and appreciate it even more now in hindsight because when I look at it for what it is, and not for what I wanted it to be, I can truly appreciate it for the fact that it really is great in its own right and does a great job in reimagining the RSE experience in a modern light, just in a different fashion from how HGSS did it with GSC.
Finally, completely unrelated to the above, but tying into Champions. I think Alder is an incredibly underrated Champion. He's not the most memorable probably because you don't battle him the first time you go to the League, but he was one of the most involved Champions we ever had, even moreso than Cynthia or Steven pre-ORAS. He's not the strongest in terms of team, but he was one of the strongest morally and had a very compelling backstory. He's a man who seeks to teach the younger generation to love Pokemon, above all else, rather than sheer pursuit of strength, and while he does enjoy battling, he more than anything wishes to bring Pokemon and people even closer together, even after he retires in BW2, and challenges Cheren's viewpoint in that sense. But he's also a foil to N: while N wishes to separate people and Pokemon in BW1, Alder sees Pokemon and people and wants them to be strong together through bonds of trust and friendship. Even moreso, while N is determined to assert his way as superior, refusing to accept different ways of living, Alder is open and tries to spread understanding even in spite of differences. This is apparent even after N is defeated by you at the castle, where even though N defeated him, Alder still tried to reach out his hand to N as a friend, and tells him that even if people cannot fully understand each other's different viewpoints, that is not a reason to reject them, which shows how strong morally he is, and is key to N's growth that we see especially in BW2. It also ties wonderfully into the theme of truth vs ideals, and how at their core, both are based on perspective more than anything, and there is no one right answer: that answer is different for everyone.
His backstory also ties into his character now, in that he once upon a time madly pursued strength with his first partner Pokemon until one day, his original Volcarona died from illness, and that changed him profoundly and made him the man with the viewpoints he has today. This really ties into how he approaches Cheren: back in BW1 he was merely concerned with strength, but Alder changed that, because he knew from his past and still regrets that he never pursued a more meaningful goal with his first partner Pokemon, and he wants to make sure Cheren doesn't go down the road he did. He is a man driven by his life experience more than anything, and much of his character is based on what he lived in his entire life. I think that makes for a genuinely great character and a really cool Champion in his own right.
(end of long-ass rants lmao)
