Gen 1 has Lt..Surge talking about the Pokemon war.
Koga is a Ninja who hides behind invisible walls.
Blaine locks his gym and hides the key in the burnt mansion where Mewtwo was created.
Sabrina is a creepy loner and beat the Blackbelts ass without breaking a sweat and the final Gym Leader is fucking Giovanni.
The Power they give off is delivered pretty subtle and implicite, which imo is a powerful design element.
They aren't saying much and their Pokemon have trash movesets, but NPCs talking about 'em almost in a fearful way.
In previous Gens, it was Like "Gym Leader x crushed my Pokemon, he's brutal", from Gen 5 onwards: "Our Gym Leader is a Hero and beloved by kids and adults alike. He also scored four Touchdowns in the 1966 city Championship game..."
The HM system added up to this effect.
If you beat a certain foe, the game considered you to be powerful enough to navigate through a whole new, lot bigger and more dangerous area.
Today is either everything linear or openworld without proper Level scaling that makes the game linear anyways.
You have a point with the leaders often having silly designs and personalities, perfectly okay for a kids game.
Maybe its just me who dug to deep into Creepypastas.
There ist a YouTube Channel called "GoldenOwl". He claims to be a game designer, analysing and rating the most infamous Boss Battles and tropes in Pokemon and other games from a creators point of view.
https://m.youtube.com/watch?v=6ibKcU8QWJA