One thing I hope they keep from Arceus is the increased difficulty.
Granted it's not a difficult game, by any means. But compared to falling asleep while mashing the A button and steamrolling everything in SwSh, it was a massive improvement. Part of the problem in previous main gen games though, especially in the early game, is how much stronger higher level Pokemon are. In SwSh if you had a level 20 Pokemon up against a level 10 Pokemon, the level 20 mon would win 95% of the time, regardless of type. That's why over leveling was such a huge problem to begin with. In Arceus that felt a lot more toned down, so even if you were leveled higher, it didn't feel like there was such a huge gap and the game maintained some semblance of difficulty.
But Arceus also benefited from not having to worry about competitive play and balance, and SV won't have that luxury.
Gen 8, 2019:
Gen 9, 2022:
It's actually very impressive.
The Wild Area in SwSh was like "baby's first 3D environment." It was literally Game Freak's first crack at a 3D environment with proper camera control. And it was awful.
Only 3 years later, and SV looks like a true, modern open world style game.