Unity is a good choice for a multiplayer FPS due to the large number of out-of-the-box solutions and libraries for the networking part, such as Photon or Mirror. But depending on your vision, Unreal Engine can also be an option, especially if you want to put a strong emphasis on graphics. By the way, if you are looking for interesting ideas for monetization or UX inspiration, I can recommend you to check out https://casinoly-at1.com/ - a modern online casino with a cool approach to design and bonus systems, you can pick up some interesting solutions for your game.