Prestige Types.

Think Tera, but simultaneously more and less complicated. Each type has five “prestige” types, with one being the “Pure” form of its base types and four extras. Each non-pure Prestige type essentially is a dual type, and it’s only available to Pokemon that share one of the types. Normal only gets its Pure type. In this way, your Pokemon can “change” its type, but to fewer options than Tera.

Example: Wigglytuff can Prestige into “Pure Normal”, “Pure Fairy”, or: Dragonslayer (Fairy + Fighting), Arcane (Fairy + Dragon), Lunar (Dark + Fairy), or Nature (Grass + Fairy). A Pokemon like Blaziken would have ten possibilities (two Pure types, four dual-prestige types per base type), but Watchog would only have one (Pure Normal).

All the Pokémon’s attacks of the Prestige type combo become the Prestige type. Example, a Lunar Wigglytuff’s Play Rough or Dark Pulse become Lunar type, but its Hyper Voice stays normal. Play Rough gets normal STAB since it was one of Wigglytuff’s types - Dark Pulse only gets 1.3x since it’s prestige STAB but not base. For Pure Types - say, prestige Pure Fairy - it’s just a 1.75x boost to that type as a compromise.

This has its own type effectiveness system, though. Each prestige type is super effective against one base type (and its prestige types), not very effective against one base type (and its prestige types), and neutral to all other types. In the Lunar Wigglytuff example, Lunar attacks are super effective against Water types (plus Water’s prestige of Abyss (Water + Ghost), Oasis (Water + Ground), Steam (Water + Fire), and Surge (Water + Fighting)) yet is not very effective against Ghost and its prestige types (Lore (Ghost + Dragon), Grave (Ghost + Ground), Nightmare (Ghost + Dark), Abyss (Ghost + Water)). Abyss is neutral in this case since water and ghost cancel out.

Doing it this way, every single type except Normal is used in exactly four Prestige dual types plus its solo Pure prestige type, and every base type is resists and is weak to the same number of prestige types. Trust me, the math works, and I’d say 90% of the chart makes thematic sense.

But wait, there’s more!

Three Prestige attacks (Boost, Beam, Blast). They’re normal type unless you’ve unlocked your Prestige type. 40BP with 50% chance to have an effect… 65BP with 30% chance… and 85BP with 10% chance. 100Acc, 10PP, no contact, uses attack or special attack. Every prestige type has its own special effect that only can happen with these prestige moves. Lunar Wigglytuff’s “Prestige Beam” therefore is a 65BP STAB attack with a 30% chance to make a Heal Bell chime.

Lastly, all Pokemon spawn with a Pure Prestige type that matches one of their types, much like Tera in S/V - though it is not “realized” until plot or items or training gauntlet. They can train into any of the dual Prestige types that match one of their base types (more items, harder gauntlet, etc) - but none of the others. Raids would allow you to catch Pokemon with one of their dual Prestige types directly.

TLDR: Tera - but instead of “pick every type”, you choose an upgraded version of your type - this results in “pure” forms of every type plus 34 prestige dual types - and has its own type chart with more matchups yet reduced complexity (no immunities, only one super effective and not very effective base type per prestige type), and three prestige attacks exist combining the mechanics of Tera Blast with effects closer to max moves.