Pokémon Stadium 3 would be great. Pokémon is all about battling, which I find it crazy many people aren't fond of it. Pokémon Stadium also had more than just battling. Pokémon Stadium games also had Professor Oak's Lab, which allowed for extra box storage and made it easier to transfer Pokémon in between games. Of course, that might not be needed today because of Pokémon HOME, but still. The Pokémon Stadium games also had minigames that the mainline games lack. While I have mentioned that the mainline games can do with a new Game Corner that features similar minigames, they were still a staple of Pokémon games that didn't involve battles. Regarding battles however, Pokémon Stadium had other challenges, such as different championship cups that the mainline games also lack and the gym leader challenges.
The closest thing that we have to Pokémon Stadium in the mainline games are battle facilities. However, those are quite repetitive and don't offer much difference in options other than selecting Singles or Doubles. Being able to win different types of challenges could net better prizes that can be transferred to the mainline games. For example, in the first Pokémon Stadium, I recall that beating the Gym Leader Castle could get you the other Starter Pokémon as a prize, as well as Hitmonlee and Hitmonchan. In other words, it allowed you to win Pokémon that you normally couldn't obtain during a regular playthrough. The same thing could definitely be done with these games. Pokémon Stadium 2 also had Earl's Pokémon Academy that allowed you to study Pokémon and learn how to battle effectively.
I don't see why Pokémon Stadium 3 can't exist today with these features and more. They could also create modes that allow us to battle previous evil team leaders, Champions, and Gym Leaders as well. I think it could serve as great bonus content for players that wish to do more battling with their Pokémon after they beat the mainline games as well. Pokémon Stadium 3 could also serve as a great venue for competitive battles as well with more rules and tiers that are not likely to be programmed in the mainline games. I can imagine it existing with much less glitches, better cries, more varied animations, and so on. I say bring back the Pokémon Stadium series and make it modern and better. If they can do the same with Pokémon Snap, then they can definitely do it with Stadium.