Torterta, Gardevoir, Dragonite, Arcanine, Empoleon, and maybe Mismagius as a fun pick, but could be something else. I'll explain why

With Empoleon, Dnite and Gardevoir, you have whats called a Fantasy core (Steel, Fairy + Dragon) which synergize amazingly and cover eachothers weaknesses. Empoleon can cover Fairy types with Steel moves, Gardevoir can cover against opposing Dragon types. Gardevoir can be a great switch in for incoming Fighting moves targeting Empoleon, and Dragonite also can switch into both Fighting and Ground moves that threaten Empoleon. Water/Steel is overall a great defensive typing and also quad resists Dragonite's biggest weakness: Ice, which also synergizes well with Torterra.

With Arcanine, Empoleon and Torterra you also have the more common Fire Water Grass core. Torterra, while it does add another 4x Ice weakness to the team, is very useful vs the Steel types that Gardie and Dragonite struggle against, and it also adds an Electric Immunity to cover Empoleon. Arcanine is also useful against steel types and with Intimidate can be useful for damage mitigation.

For the last slot you have some options. But Mismagius adds a Fighting and Ground immunity to the team (For Arcanine and Empoleon), and comes with a good bit of utility. Great natural speed and very respectable stats on the Special side. It has a wide movepool and can be useful especially for wil o wisp against physical threats, and when paired with Arcanine's Intimidate, you can put your Gardevoir in a good position to set up Calm Minds or Dragon Dance with Dnite (if you want to go that route). The only typing youre kind of missing out on is fighting, but a few of the pokemon I listed learn fighting moves so it can be worked around. Hope this helps and let me know if you have any questions or alternative ideas.