Some of my relevant thoughts from the "Trials vs. Gym" thread (I'm currently on a mobile interface, so some quotes from other users over there didn't transfer properly):
I think that I would rather have Gyms; Trials are essentially the same thing, except that you don't get a sigil/token of accomplishment for the non-Grand Trials (the Z-Crystals don't count, since they are equivalent to Gyms' Technical Machine (TM) prizes, and they don't look like Badges), and the Totem Pokmon are not under the direct ownership of the presiding Pokmon Trainer (whereas a Gym Leader's Pokmon would be under such ownership). So I guess Trials/Trial sites are "Gyms: with puzzles, with the Gym Leaders 'replaced, sort of' by a stat-boosted/augmented wild Pokmon, mostly without a Badge, with Z-Crystals in place of TMs".. If one considers Trial Captains as Gym Leader analogues and Island Kahunas as the Elite 4 analogues, then League structure is also kind of changed - instead of "8 Gym Leaders, then 4 Elite Four members", the order is "1-2 Gym Leaders, then 1 Elite 4 member. Repeat that process 4 times.". It's a slightly interesting mix-up in terms of order, but I'd rather have Trial Gyms - the Pokmon are directly under a Trainer's ownership, everyone doesn't just necessarily rip off Petalburg City Gym's mid-battle stat-boosting gimmick, and you get a symbol representing your fight (come to think of it, here it's weird how in Alola "Elite 4" give the "Badges" rather than the "Gym Leaders" XD). I like having Gyms mixed in with the overworld like Alola did, though. I also like Alola's Trial Captain order as a mix-up.
Also, if I'm not mistaken, the Totem Pokmon implementation in Trials means that we don't get a proper "Gym Leader battle theme" analogue? Apparently, "Vs. Totem Pokmon" is just a remix of "Vs. Wild Pokmon (Alola)"? If so, that's another reason I'd like Gyms or Trial Gyms, since we'd get a unique Gym Leader/Trial Captain battle theme like most, if not all, of the past regions have had.
Interesting; under this perspective, that would make Alola seem underwhelming with only 7-11 "Gym Trainers" region-wide and only 4 Gym Leaders region-wide (and 4-5 "Elite 4" members (due to repeats), not including Champion/proto-Champion Kukui..?) All of the past regions at least have somewhere between 20-100 Gym Trainers region-wide, 8 Gym Leaders, and the 4 "Elite 4" members..
Despite Alola technically having an actual "Elite 4" separate from the 4 Island Kahunas, I personally considered the Trial Captains as Gym Leader analogues, and the Island Kahunas as Elite 4 analogues.. The actual Alola Elite 4 was unnecessary, in my opinion, since 3 members were lazily rehashed, and most Leagues just have one League battle against each League member (sans Kanto and its Giovanni+rival shenanigans).
Kahili is in an odd spot; without the other 3 rehashed Trainers, she seems to fall somewhere between Island Kahuna (although that would mess up the island numbering, unless you include the artificial island), Champion (Kahili fits due to being a former island challenge Champion, although I rather like Kukui referencing the Generation 1 Champion beta element of Professor Oak, plus the position ties in with him researching Pokmon moves), and facility head/Frontier Brain level (maybe a good fit for Kahili, since she definitely has skill but her current placement is somewhat of an anomaly)..
As for what I expect in
Pokmon Ultra Sun/Ultra Moon, due to the alternate story thing, I feel like Trials will be retained from
Pokmon Sun/Moon anyway (hopefully with Stamps for the non-Grand Trials); maybe some Gyms will be present as a bonus.. I kind of hope that the unmanned Trial Site(s) (1 that was active with a Totem Pokmon in SM, and 1 that is not active in SM but was used in the past, if I remember) will get used (maybe with Ryuki).. I feel that Trial Captain and Island Kahuna types will either be retained or mixed up. I hope that as many of the 18 currently-existing types can get used in some way (with consideration of Guzma as a type specialist who parallels the Trial Captains and Gym Leaders - as in, if he is still Team Skull's leader, his type should not conflict with the type of a Trial Captain or Island Kahuna). Somebody in the "Alola League" definitely needs to use the Ice type, since it went unused in SM.
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I think that Trial Captains and Island Kahunas will be retained and rearranged from SM to US/UM. However, I do like everyone's characterization from SM (especially Mallow's <3), although Hau seems a bit too goody-goody from what I've seen of him. But like others have said, Hau somehow surpassing his grandfather Hala (and achieving his goal) would be neat (in spite of the alternate story thing).
It really irks me that the actual Alola Elite 4 re-uses 2 Island Kahunas and 1 Trial Captain (even if Kukui is scrounging around trying to form a "League"..the island challenge trial hosts make good League members as they are) - and why in the world are Island Kahuna Nanu and/or Island Kahuna Hapu getting shafted of an Elite 4 spot thanks to Trial Captain Acerola, when Nanu and Hapu are clearly stronger than Acerola? It makes no sense at all..
I feel like Kahili is a great character (although maybe too reminiscent of Winona), but her position should be either:
-altered to the 5th Island Kahuna of Alola's artificial island,
-retained as an Elite 4 member, provided that the 3 other "Elite 4" members are replaced with new people (otherwise, if the "Elite 4" is just going to repeat people and result in redundant fights, you might as well excise the Elite 4, thus leaving the 4 Island Kahunas as Elite 4 analogues)
-act as proto-Champion/Champion in place of Kukui (my least preferred option, since I like Kukui as Champion)
-converted to Kahili being the true facility head for the Battle Tree (so that someone can logically run the place when Red and Blue end their vacation in Alola and go back home to Kanto).
Nevertheless, Kahili's golf course thing needs to be expanded/implemented properly (if she even is still associated with it in US/UM).
I feel Kahili was an unnecessary inclusion as well, and you also have to notice she was a former Island Challenge Champion who only got interest to return to Alola because of the league being established.
If there's any member of the current Elite Four who I think could be dropped, I'd say it be her since we never really see her much outside of the league unlike Hala, Olivia and Acerola who we all met prior to them being in the Elite Four. And hey, the Indigo League did change Elite Four lineup between R/B/Y and G/S/C so it wouldn't hurt to try that formula again.
This stance seems surprising to me; if anything, I thought that Kahili is the one person who deserves that Elite 4 spot rather than just being dropped, since she's the only one who demonstrated enough skill and trounced everyone else, including her Elite 4 colleagues (or whoever was active as Trial Captains and Island Kahunas when she undertook her own island challenge in the past). She just needs to be fleshed out a bit more, in my opinion..
I'm really thinking that Gyms will have been constructed on these sites meaning USM will have a fully functioning Pokmon League.
Elite Four could stay the same, but it's likely that one or two of the members will be swapped out in place of new ones. As for the Gym Leaders, I'm hoping we get a mix of older and new characters, perhaps from the Battle Tree or ones we're already familiar with from Sun & Moon. Mina having a Fairy-type Gym for example would be awesome.
Nice to finally see those purported empty Alola sites; they would indeed make great spots for Gyms or Trial-based Gyms.. It would be interesting to see Ilima and Mina switch types - however, I really want to see what were the original intentions for the type specialties of Ilima and Mina, since their roles being reversed makes too much sense...!
Guzma being a Bug-type Trial Captain or Island Kahuna (or even Elite 4) would be interesting, as would Ryuki being a Dragon-type Alola League member.. I'm fond of Mallow representing the Grass type.. If still Champion/proto-Champion, Kukui should retain his type medley.