It depends a lot on how they address it. It feels gross to say it, but Alola was on to something with the trials because of the exploration factor tied to the trial sites.I agree that they should be mini-dungeons of some sort, but what exactly that means in Pokémon is pretty vague. The series has never had great ways of interacting with the actual environment (HM blockades like Cut/Strength/Rock Smash are about it, and of those, there's only potential puzzle elements when Strength is involved). They can incorporate small setpieces for puzzles, and they've done this, but by design these have to be simple to interact with and understand, since they're not going to be used outside of this one instance. I suppose I'm not sure how "interactive setpieces used only in one building" is less of a gimmick than what SV usually did.
Maybe they didn't succeed in execution, but I liked most of SV's gym challenges as concepts because they forced you to interact with the world in some way, usually tying it to local theming. If anything, given how mini-dungeons/caves/whatever locations in Pokémon you consider dungeons usually go, forcing you to explore a little fits that design better than a goofy little puzzle.
Again, they want to make these challenges a fun diversion, but they also need to make them simple enough to not cause problems for younger players. I am a lifelong gamer™ so I can't say how difficult other sorts of puzzles would be. I vaguely remember not solving Chuck's Strength puzzle in GSC first try, and actually being briefly stumped by it, despite it only being 3 boulders (and I was around 9 or 10 at the time, so not particularly young). I think it is a loss that they set up all this work for gym challenges only to use them once and discard that effort immediately after. Could be fun to reuse some of the assets for a minigame somewhere else, or to have a tougher, redone gym challenge using the same system for potential Gym Leader rematches.
- Katy had the Olive Roll, which you can mess around with outside of the Gym, and is thematic to Cortondo.
- Brassius's Sunflora hunt fits in with his Sunflora statues, and the city's generally impressive greenery.
- Iono didn't do this, really, but she has her own theming and it plays along with that. Not a particularly inspired challenge but it was okay; maybe they held back for hers because Levincia is the second biggest city in Paldea.
- Kofu's intended challenge is never seen; instead we trek to a neighboring city to participate in its auction minigame, which can be used to obtain rare items.
- Larry's challenge works a bit like an investigative puzzle. I think some of the location's clues are not as clear as they should be to find, but it still made players interact with 3D space.
- Ryme's challenge is mostly just a way for players to try and get a feel for double battles, because they otherwise don't exist in SV. There's some poor rapping dialog interspersed to fit her theme, which gratefully doesn't impact the fight except for stat boosts from the crowd's cheers (is this also meant as an introduction to raid cheers?).
- Tulip's ESP is a bad acronym pun and the easiest Simon Says minigame ever, and doesn't really tie into Alfornada or her character at all aside from the aforementioned pun (and I guess her friendship with Dendra). Maybe it's intended to show off camera poses and expressions?
- Grusha's challenge is a very easy snowboarding minigame. It doesn't add anything, but it ties in with his gym's remote location and his history with the sport.
If they improve the exploration phase of the games, there's more room for fun mini-games or things like that, even if I don't like how gym trainers are an afterthought nowadays.
If things stay as is, then I'd rather have the Gyms fill the dungeon void because they can make it a more controlled environment, and perhaps, something good can come out of it.