While there's a bit of hindsight speaking here, looking back I think it's pretty cool that two of the members of Ash's very last team, his Journeys team, were Dragonite and Gengar.
They are both Gen 1 Pokemon, and Ash is from Kanto and started with the original Gen 1 anime season, ending with Journeys, which was multi-region, and as such Ash's team was a mix of two Kanto mons, Lucario, and two Gen 8 mons.
But the fact that the two Kanto Pokemon he newly acquired on his last team were Gengar and Dragonite is neat in a way because of what those two Pokemon represent, what their role was in the original Kanto games, and how it represents the great and powerful Trainer Ash is now.
His original Kanto Pokemon in the Kanto season of the anime was, in addition to his first partner, Pikachu, a group of early game Pokemon. A Caterpie that eventually evolved all the way to Butterfree, a Pidgeotto that evolved into Pidgeot, Bulbasaur, Charmander (evolved all the way to Charizard), and Squirtle. These are all introductory level Pokemon in the original Kanto games. Pikachu was an unconventional first partner for the anime's own storytelling reasons, but in the original games it's an early game rare spawn in the very first dungeon of the games. Bulbasaur, Charmander, and Squirtle are the three traditional starter Pokemon that new Trainers obtain as their first partner. Caterpie is an introductory tutorial Pokemon, evolves all the way very quickly in an intuitive way (insect metamorphosis process) to Butterfree, in a way that teaches the concept of evolution to a new player, and then becomes a useful ally for a while before falling off. Pidgeotto/Pidgeot is one of the first early game Pokemon you will ever meet.
These being his original partners reflected how Ash was a new and inexperienced Trainer during the original Kanto season. As such, he had a squad that embodied what are essentially both from a Doylist and Watsonian standpoint, Pokemon best suited for beginners.
Later on, he ended up with Lapras and Snorlax, with some other odd catches in Muk, Kingler, and 30 Tauros before that. These are all mid-game Pokemon, and in particular Lapras, Snorlax, and Tauros are relatively strong mid-late game Pokemon you obtain in different and unique ways but are very much strong and good Pokemon in their own right. These came later in his journey, as he wasn't anywhere near as strong as he was towards the end of his tenure as the anime's protagonist, but by the end of Kanto and towards Orange Islands (where he caught Lapras and Snorlax and started using one of his Tauros) he had become a more experienced and capable Trainer. As such, he had Pokémon suited to where he was.
Now, many generations later, in Journeys, he gets two more Gen 1 Pokemon, and as I said, those ended up being Gengar and Dragonite. At this point, Ash had become the Alola Champion, and with this series, it was all about his meteoric rise to becoming the
World Champion, the most powerful Trainer and battler in the world. Aka, a Champion level Trainer.
And why is it fitting that two of the members of the team he used to become the World Champion are Gengar and Dragonite? Because what they represented in the Gen 1 games and Ash having them now embodies that he has truly become one of the best Trainers.
Gengar and Dragonite were two of the most unique Pokemon in RGB, in many ways, and were perhaps the most elite of the original 151 aside from Mewtwo. They had unique typings given exclusively to them: Gengar was a Ghost-type, and Dragonite was a Dragon-type.
Gengar was the final form of a unique line in Gastly and Haunter, and their "being a Ghost" schtick made them unique ethereal enemies that had a lot of qualities that put them a cut above the rest. You couldn't face them without a Silph Scope. They were only in the Pokemon Tower, and they were immune to every single Normal and Fighting-type move, resisted Poison and were immune to the Poison status, and their high Special made them take a lot of special moves very well, especially Grass moves which they resisted, which meant they needed to be defeated in unconventional manners. Most Trainers who had them there were possessed by the Gastly/Haunter they used, not actually owning them. The only one who does is Agatha, and she comes at you with the mighty Gengar, who is the final endpoint of the unique Gastly line, and she was the second to last Elite Four member, making Gengar one of the final boss fights.
As far as owning one, Gengar's uniqueness was reinforced by it being one of the trade evolutions, requiring that extra effort for a Trainer to own one. While the other three had different qualities to boast, Machamp being the ultimate physical attacker with superhuman muscular strength, Alakazam being the ultimate special attacker with a hyper-intelligent brain capable of immense psychic power, and Golem being the ultimate physical defender with a rock hard shell, Gengar was the ultimate result of a Ghost that required unconventional techniques to defeat, and those who had the fortune of owning a Gengar were given a powerful asset that was untouchable by most common enemies and had plenty of great tactics in addition to their speed and special power with a useful set of TMs and disruptive tactics such as Hypnosis+Dream Eater, Confuse Ray, and steady damage in Night Shade to wear down enemies.
Dragonite on the other hand has the Dragon-type and has the highest Base Stat Total of every non-legendary in Gen 1, at the time 500, and post special split a solid 600. It was the ace of the strongest Elite Four member in Gen 1, Lance, and Dragonair and Dragonite resisted all three starter types, aka the elemental types, making them impervious to Venusaur, Charizard, and Blastoise without coverage, while having strong bulk to take non-STAB Normal hits and hitting back hard with Hyper Beam, coming off an immense 134 Attack stat. Being under the ownership of the most powerful Elite Four member served to make it a penultimate boss and highlight its elite status.
Ownership wise, Dratini was a rare spawn in the Safari Zone specifically, and was hard to catch with the Safari Zone's rules. From there, it levels up very slowly and is weak, and evolves at a very slow pace all the way to Level 55 as Dragonite. High investment, high reward, but the end result was one of the strongest Pokemon in Gen 1.
The fact that Ash owned them during his time in Journeys, with his rise to the top as one of the world's greatest Trainers in that regard, in my eyes indirectly basically highlighted where he is now and that he is truly one of the most elite Trainers, not just among those from Kanto, but in the world, especially for the boy from a small town in Kanto.
I think it was a cool inclusion to add to his final team in the series that was about his rise to the top.
