Unreal Engine 4 (comes with the Editor) is completely free, and while similar but a bit different to Unity or FC 5 Arcade, it is very powerful, and Free Games are released like "Subnautica" regularly.
I am a mapper, and have over a decade and a half experience in 3ds max, Photoshop, and a good handful of years in Mudbox, i am planning an FPS title and once i build the main characters (only started the first one, recently retired from Indie Dev'ing) i will present the Design Document and Assets i created to the Epic Forums and Recruit Talent to make the game a Demo/Alpha, and hopefully a Early Access Game on Steam.
What i suggest is you try out Unreal Engine 4 (free to publish games as you wish, only pay Royalties if you make $250,000 or more on the game, and only 5-10%)
If you would like to be a Map Designer, i would love to work with an experienced mapper such as yourself, i will add you on Uplay, and hopefully we can talk about building a great FPS that is Mostly a Story-Telling FPS with similar mechanics to FC5, with driving, exploration, and story-telling (But we will allow "Skippable Cut-Scenes

) but vastly ddiffernt setting, tried a Release of the game's concept (which i own) and we made it to Demo Release on ModDb.com, but never finished it due to the Main Programmer leaving for Military Service and 2 of the mappers still in College, without a Main Programmer, they felt it was dead and left the team.
Thanks for all of your maps, i think some even got ported to PC, i don't remember all the names of the maps i've played or their creators, but i've had a blast creating maps in FC 5 Arcade, even thouhg there are no plugins for creating your own assets in Blender r Milkshape or 3ds Max or Maya.
[EDIT] i tried adding you on Uplay, i know cross-platform isn't a thing, but i hoped it would at least give me a friend on Uplay, feel free to add me on Steam, Same Name, and we can see about putting that good talent to use for fun (and profit, hopefully) And i don't know if you have a PC but the editor is pretty straight-forward and having someone build Custom Assets that you can simply place in and even brain-storm about the direction you would like to go and modifying the theme to your liking would work with me, i'm very flexible, and have good working knowledge of making props and assets for mappers, did so since Unreal Tournament 2k3 [EDIT]
Some recent models i've created just to keep my skills un-rusted, that aren't under NDA:
3rd person (in-game model) of a Counter Terrorist Unit Agent:
8,000+ Polygon Basic AK-47 with iron sights aligned for CQC:
