Shatterline is a free-to-play arena FPS with a roguelike co-op mode (Expedition) as well as several traditional PvP modes such as TDM, Conquest, Plant-the-Bomb, and Escort.
Players can choose from 8 unique Operatives and an armory of 25 weapons that can be customized via attachments.
https://store.steampowered.com/app/2087030/Shatterline/