I know the slate system in of itself is controversial but the staff themselves have noted on numerous occasion that they are aware the end-game is lacking, and right now the only end-game we really have is shiny hunting. I think sadly with no IVs/EVs/Ability separation and natures that there isn’t much of an end-game other then shinies, that said this is an idea of something that could additionally be added.
Currently slates offer no real secondary purpose, you acquire them and then you fuse them into your mysterious part and you can temporarily call on the power of a legendary pokemon in a dungeon. Slates have their own issues but this is not about that, this system would also have an extended goal by introducing shiny slates as it’s kind of necessary in order for this to work as a true end-game, without the possibility of shiny legendaries then there’s not… too much once you complete the challenges here. The idea of restricting these challenges to a couple tries a day is to prevent people from exhausting it all at once.
Back to the idea: The idea is simple, throughout pokemon there have always been “trios” connected to some other pokemon. Instead of giving every single legendary pokemon a dungeon, or putting their slate in an end box or something like that… this is a simple idea of an isolated arena, let’s say it could be somewhere special like a special area that you have to use a HM to access or something, or just an obscure part of the map between regions.
This area would not lead to a dungeon, it would lead to a gate that would react based on the three(+) slates in your inventory and that would be the trios. It would be a fight you could attempt a few times a day (3-5), drops would range from unique ones to the legendary(maybe family items?), general good items and of course their heart slate… and if we have shiny legendary possibility, then you have a 50% chance of getting their shiny slate or a guaranteed regular slate. Maybe make these arenas unable to be entered unless your inventory is minimized, or just make it so these bosses are immune to slumbers/orbs in general, which could encourage the use of buff orbs instead.
Anyways - Bold for ones currently doable, italics are behind already HC-locked slates.
Moltres, Articuno, and Zapdos Slate = Challenging Lugia.
Entei, Suicune and Raikou Slate = Challenging Ho-Oh
Regirock, Registeel, and Regice Slate = Challenging Regigigas
(Exception idea: Azelf, Uxie, Mesprit, Giratina, Palkia and Dialga slates for fighting Arceus, could maybe integrate all the type plates into Arceus’s drop pool)
Virizion, Cobalion, and Terrakion slates = challenging Keldeo.
Tornadus, Thunderus, and Landorus slates = Challenging Enamorous
Zekrom or Reshiram slate = Able to challenge Kyurem and maybe it takes on the enhanced state of which slate you use to challenge it?
Yveltal + Xerneas slates = Able to challenge Zygarde
Honestly some of these challenges could be replaced with a genuine dungeon, but I feel this is an easier way to get some of these slates out of HC purgatory without needing to map out and design a full dungeon with recruitables, enemy wild pokemon, item spreads, etc.
It’s just an idea and I feel it could give us a little something more to do, besides just hunting shiny recruitables. Even without shiny slates it could still add something, especially with the gated limits of tries per day.
Side idea associated with this idea: A rare chance for legendary pokemon to spawn as shiny and when they do, if you defeat them you either get their slate or their shiny slate.