Normal - Delcatty. It has a really interesting ability in Wonder Skin and a good support movepool. It's also a cute purple cat. Give it the Fairy dual typing it should have already gotten, update its defensive stats.
Fighting - There's not really any Fighting Pokemon I like who needs updated stats. Meinshao could do with a + 10 in speed and a priority move.
Flying - Togetic. If Togetic had good HP (+30?) you might justify using it with Eviolite over Togekiss, which I would love to do since I think it's cuter.
Poison - Dragalge. I really think all this one needs is some half decent HP (85?) and Recover. Recover + Toxic Spikes + Sludge Wave/Protect + Draco Meteor.
Ground - Sandslash. Honestly, I'd love for this guy to just get an evolution. Because its stats would have to have a lot added to them for Sandslash to be useful.
Rock - Archeops. Actually, don't update it's stats, just remove it's horrid, horrid ability. Why would they even do that?
Bug - Vivillon. Vivillon's not actually completely useless in Doubles as it stands. But I just imagine how great it could be if it was speedy and bulkier. But considering its type, even that might not save it from depending on a Focus Sash. Unless it got like +40 on its defenses. Why couldn't it have been Bug/Fairy? It would still have weaknesses, but it would also have more resistances.
Ghost - Banette / Mega Banette. I love Banette. I love mega Banette. They should have built on its defenses and given it a recovery move. Prankster, 64 HP and no recovery?
Steel - None. The only steel Pokemon that I even remotely like is overpowered!
Fire - Delphox. As it stands, Delphox is a crappier version of Victini. Fire/Psychic offensive, just without the good ability and move. Give it Drought. Give it ridiculous speed. Make Mystical Fire a priority move. Just do something so I can use it, because I love its design.
Water - Dewgong. I love it's sweet little face and simple design. I hate that it's been subpar from day 1. I don't even know how it could be improved. It has terrible defensive typing and 3 defensive abilities.
Grass - Sawsbuck. What a unique Pokemon. What a shame its crap. If it's Attack was higher and it had a more versatile movepool, you might justify using it on a sun team.
Electric - I DEMAND FOR DEDENNE TO BECOME OP. I love that little Pikaclone. And it's typing? Pretty solid defensive and offensive typing, in my opinion. Of course they had to make it complete crap.
Psychic - Meowstic-F. My favorite shiny. She has a two good ability options for an offensive Pokemon, and she has the movepool. Now just give her some monster SpAtk and Spd.
Ice - Aurorus. Honestly, there's nothing you can do by just adding to its stats. I think it's only possible use could be as a glass cannon because of it's crappy typing. That's all anyone could possibly due to make it useful. It has a good ability for being a glass cannon in Refrigerate along with access to Hyper Voice.
Dragon - Flygon, of course. I'm actually not even a huge fan, but it certainly deserves a big update!
Dark - Liepard. Am I wrong for wanting to slap a Fairy dual typing on this and some decent bulk so it can be effective with Prankster? I think Mightyena deserves a mention here too. Major update it's horrid stats or give it an evolution.
Fairy - Florges. Florges is without a doubt still a good Pokemon, but if only it had slightly higher speed and a better ability. I think there are plenty of good abilities that suit Florges. Sap Sipper, Cloud Nine, Chlorophyll, Effect Spore, Harvest.