i have this project (not dead , on pause, will resume soon)
Like you, i came with the idea of collaboration . But the reality i learnt is : nobody will make the game for you.
So i’m afraid you gonna have to create you game prototype first. Show it, then maybe some motivated people are going to show up to join your project (i only know 1 person @Fred_K.S having succeed setting up a little team … his project is going to be released #soon) … but at that point, when it gonna happen, probably you won’t gonna see the real adding value they bring on the table 
look at Jobs offer on the forum, you can post it there. https://blenderartists.org/c/jobs/paid-work/53
Otherwise, i recommend to find someone in the UE/Unity community.