A classic FPS with an easy to use Level Editor and that is cross-platform? YEP! Check out Prodeus at PAX West 2019!
http://www.prodeusgame.com/website/index.php
About Prodeus:
Prodeus takes the look and feel of the iconic 90’s shooter and reforges it in the crucible of Moore’s law. More action, more explosions, more blood, and more over the top visual effects. At its core, Prodeus is a fast-paced action shooter with lots of environments to explore and secrets to discover.
They are also developing a powerful and intuitive level editor that makes creating unique, hand crafted experiences, fun and easy. What’s more, this editor will ship with Prodeus along with the community tools that make it easy to share your maps, as well as browse, play, and rate other community levels.