Maybe the spawns should be less about the theme , and more about the synch. Not gonna lie , most of the guild island spawns are absolute rubbish , guild island is definitely not the reason guilds compete for the top 3 spots ( And the fact that a guild can get into guild island using a ton of mysterious tickets is an absolute joke ).
The only good spawn guild island has ever had (to my knowledge) was when the cave had these 3 pokémon in the same spot : sneasel , gligar , and gible . And for one reason , all of them share a common nature which is jolly. And from what i've witnessed , that actually gave people the incentive to hunt on guild island.
You need to give guilds reason to try to reach the top spots , aside from the prestige and fame . Having spawns revolving around nature will make guild island a success for sure . This might be a bit too overpowered , and its still just an imperfect idea , but something along thoes lines will definitely make guilds compete for the top spots.
Examples : ( i'm not saying have all these pokemons in the same spot , 2 or 3 per spot would be perfect )
-Jolly spot : Gible , Sneasel , gligar , beldum , buneary , mimikyu , charmander , drilbur , jangmo-o , meditite
-Naive or hasty spot : Chimchar , froakie , honedge , ralts
-Timid spot : Snivy , charmander , abra , larvesta , treeko , deino , houndour , magnemite
-Adamant spot : mudkip , pawniard , dratini , azurill , hawlucha , mawile , larvitar , corphish
These are the pokemon just on top of my head , you can be as creative as you want with the spawns. And implementing the rotating spawns idea with this would make this even better. Something along these lines will definitely give guilds a reason to aim for guild island.