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I really want a mainline game where they bring back the Shadow mechanic.
The games could have the usual regional pokedex and exclusives.
The Shadow Pokemon could consist of UBs, legendary/mythical pokemon, starter pokemon, the other games wild exclusives and pokemon that don't show up in the region normally.
That way you could play through the game and never have to actually catch a shadow pokemon because you can rely on the usual wild encounters (unless you wanna catch em' all), but you also have the choice of saving and using the shadow pokemon.
They could even allow you to find the original trainer once you've purified a pokemon. Maybe get a reward if you choose to give it back, maybe an egg of that pokemon line with a special move.
I just felt so much more engaged in the story when I wasn't simply mowing down the grunts/etc, but trying to take back the shadow pokemon from them.