Definitely agree. One of the biggest hurdles when it comes to making full-team boss fights difficult is making the AI resilient against setup sweepers, which is near impossible to do universally without some very clever switch tactics and/or putting highly specific countermeasures on almost every team.The problem is, fundamentally, there's no good rule. AI isn't good enough to Dynamax at the right time in battle, because that depends heavily on PC team comp, NPC team comp, etc. So you either DMax(/mega/Z/etc) immediately, which risks the PC being able to easily counter, because they led with a type advantage or a sacrifice, and even if the player doesn't easily counter, it turns the battle into "beat the lead by sacrificing 3 mons, and then sleepwalk through everything else". Or they DMax last, which is easily countered by various setup against the mons that don't DMax. My no-Dmax runs usually turn into Sticky Web+dual screens against their 2nd-to-last mon, which isn't particularly interesting. It's a tough challenge, and I don't blame GF for having trouble finding a good solution for it.
Alola, with it's Totem Battles, was a welcome change. Lean into the "single boss enemy" vibe, once the player has beaten that, it's over, and just toss whatever benefits you want onto that boss. There's room for improvement, to be sure, but trying to beat that without overleveling and without Z moves was an experience.
Totem battles flip this on its head really effectively, but in doing so make themselves vulnerable to a strategy of stacking as many debuffs as possible and then whittling them down (or even PP-stalling them lol). Challenge runs in Alola are mostly about identifying what combination of Sturdy/Prankster/Thunder Wave/Baby-Doll Eyes/Toxic/Protect/etc you have access to at any given point in the game, which is super refreshing in the broader context of Pokemon games, but can get a little tedious in a particular playthrough after a while.
Yeah the rotating roster seems like a sensible option, although it could be tough to balance the difficulty between all the possible rosters given that monotype trainers often rely heavily on one or two Pokemon to cover a major weakness.I feel one relatively cheap way to make ""bosses"" more challenging would be them just not having a fixed team and instead always pick a random X out of a bigger pool + their ace, kinda like special trainers worked in the Battle Tree and Leon in the Battle Tower.
To some degree it even works "in lore", because the anime (as well as the fact rematches exist in some games) shown that gym leaders in fact do have a wide selection of Pokemon available, and pick their team based on the opponent's experience in order to provide a "fair challenge".
However there is still the issue of the monotyping... that's basically a huge gate in making team compositions that are at same time synergic and not facerolled by a single pokemon. Maybe they should take a page off the Smogon monotype meta, though even there, for obvious reasons some types are basically unsalvageable and just matchup dependant anyway.
It's a shame that the type chart is so unbalanced when it comes to monotype. Water is pretty much the only universally strong option for in-game monotype, since other competitively strong types like Steel and Flying would likely struggle when piloted by AI and without access to certain specific Pokemon (e.g. Heatran), whereas Water can just set up Damp Rock rain and then take advantage of the million buffs that rain provides, while having far fewer checks than other types and the ability to power through one or two Water resists pretty easily.
Team themes besides monotype can be really cool if executed well, but when I've seen it done in fangames etc. it often feels like the theme was reverse-engineered from the desired team, or at the very least that the creator's conception of the theme was very specific in a way that few players are likely to vibe with.Maybe your usual psychic leader has a sunset theme with fires and fairies in balanced mode, which then are finetuned to be a screens team in challenge mode
Like, I know this is just a briefly-stated example, but I'm struggling to think of Psychic/Fire/Fairy mons that specifically evoke 'sunset' as a theme for me.