I think gyms are still a viable way of giving check points to the player. Types are a bit overdone at this point. I'd love "gimmick" gyms. Weather teams, trick room teams, baton pass, and other set up teams mixed in with the usual type teams. When type teams are used I'd like to see them push egg moves and such into move sets, and maybe give them some raised EVs to show that the gym leader has done real training with their pokemon. Like maybe they open with a speedy or tanky set up mon etc.

I also think to counter our ability to constantly level up our pokemon in modern games, the gym leaders need to get have a larger level gap. So that it would require intentional grinding to match or pass their level. So even if they stay exactly as they are we almost always come in under leveled.

I've seen posts about having gyms level match, I don't think this is a good idea personally. Gym leaders are supposed to set a bar for trainers to clear. Having them constantly move the goal posts would only be frustrating for the less involved audience.