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Lillie not having one would make sense but the rest ought to have had them being they're all Trial Captains in the games. Mallow's character development has been non-Pokemon related regarding wanting family approval(first father in Episode 39, then brother in Episode 72 and finally mother in a very sad Episode 108) and the Alolan Tournament could have been the final phase in proving herself as a Pokemon Trainer. Lana's growth has primarily been with training and evolving Popplio, although the Z-Ring/Z-Crystal arc on Akala Island can't be ignored. Sophocles' growth has been with Charjabug for the most part(like has he really had anything meaningful with Togademaru?) and Kiawe's with Marowak and his grandfather's Charizard(even battling Brock was a growth moment).
TLDR: I would have done things differently in SuMo. Acquiring Z-Rings is indeed a mechanism for character growth but frankly they ought to have had them from the get-go. Its just in doing so, Ash would have been limited in what Z-Crystals he would have had. Kiawe obviously has Fire but he now has Flying, Lana has Water(like she should) while Lillie has Ice and potentially Fairy(although Mina kind of showed that off if memory serves). With Ash getting Electric and recently Steel, only Z-Crystal that makes sense for Sophocles is Bug thanks to Vikavolt. Doesn't make any sense seeing as that should be reserved for Guzma but eh what do I know. Finally Mallow should have Grass but they gave that to Ash when he completed Totem Lurantis. Psychic is still a possibility if she recruits Oranguru but aside from that, what else could she do unless she goes and does Totem Lurantis herself. I understand the Anime has considerable freedom but to deviate this much from the games doesn't feel right to me.
Meh anyways... FTE should be starting up in the coming weeks. If you want to join in, now's your chance :)