I just finished Colosseum
(mostly, I still gotta finish purifying everyone before trying to get Ho-oh) and
holy shit the game is fun. It wasn't particularly hard, which surprised me since I had imposed an extra challenge on myself by being extra-stingy with TMs*. Only real instance of grinding was picking Plusle up to speed in Pyrite
(time that I also spent purifying most of the Shadow Pokemon in Pyrite which is something I would have had to do anyway) and I found myself hardly ever using items in or out of battle. Since most trainers can be rematched, and their dialogue and teams updated through out the story, fighting them to keep my levels up didn't feel like grinding at all; it was just taking the game slow and not rushing
(NPCs update their dialogue so frequently in this game, it's great).
But while it wasn't challenging, having everything be double battles allowed for incredibly engaging synergies within my team. It seemed like no matter which two Pokemon I had out, there was something interesting that could be done between them, and I kept discovering new ways to combine them as the game went on. And that was just what I could do with the tiny pool of options the game gave me! A mainline game dedicated to double battles, where moves like Helping Hand, Follow Me, and Fake Tears are usable on more than just a handful of Pokemon, would be so cool, and make using support Pokemon so much more viable.
*My ultimate goal† for Colosseum was to advance progress in beating the Emerald Battle Frontier. This mainly involved the pinch berries which I also still need to get, but it also meant hoarding any non-renewable TMs in case there was no way to breed them onto the Pokemon I wanted, abstaining from using the legendary beasts in case I wanted to EV train them, and snagging a male Heracross because you need to breed if you want Megahorn before level 50 and I ain't grinding the Safari Zone for that shit.
†besides filling out more of the Pokedex