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Let's face it: hate it or not, gen 7 did an excellent job making a vanilla, main series Pokemon game difficult without a hard mode. No, they're not hard games, but they are hard Pokemon games.
A big reason why Pokemon games are easy is that the player is just too powerful with too many tools at their disposal. You have the completely unfair knowledge of what Pokemon they're gonna send in next (Switch mode), you have as many potions as you'd like, you have the generational gimmick (Amie/Refresh bonuses, rotom boosts, etc.), and you have of course your own skill.
Gen 7 managed to suppress these advantages a bit thanks to 2 v 1 battles, which inherently give the opponent the advantage, and through auto stat boosts, which should be self explanatory. They also give some trainers EV trained pokemon, which helps with the difficulty of trainer battles.
Now, the question is, what can gen 8 possibly do to match gen 7's difficulty? We know GF isn't ever gonna implement a hard mode again, and totem-like battles might be unique to gen 7.