You will need games that have crossplay between PC and Xbox One so you can play together across different platforms. Multiple people have suggested Left 4 Dead 2, but that doesn’t have crossplay. Most of the shooters I personally play don’t have it, but there definitely are shooters with crossplay.

Here are my recommendations that have crossplay:

• Borderlands 3 — Collect wacky guns, travel the universe, and shoot bad guys together. The previous games in the series don’t have crossplay.

…Yeah, that’s it for crossplay shooters I recommend. For crossplay games that aren’t shooters:

• Overcooked! All You Can Eat — Chaotic co-op cooking. Work together to prepare, cook, and serve food in increasingly absurd scenarios: in the middle of the highway, on an iceberg, in a hot air balloon that crashes into a different restaurant.
• Ultimate Chicken Horse — Platformer where you build the levels together and then race to the finish. You only score if someone died, so you need to make the level extra dangerous.
• Moving Out 2 — Goofy co-op game where your group plays as a ridiculously reckless moving company. Carry furniture from the house and shove or throw it into the truck. No one will notice if you break all the windows.

If everyone is on PC, things will open up a good bunch. Old-school networked games generally still work. You can go FFA deathmatch in your old favourites or in newer arena shooters, like Warsow or Disco Dodgeball.