Godot is coming up mighty quick. I've been favorably impressed. Also, if your concept is 2D, it's way beyond the 3D-centric game engines for that task: might not be the question being asked, but it's worth a thought.
Maybe keep an eye out for S&Box, the successor to Garry'sMod.
It's not out yet, but uses the Source 2 engine and C# for modding, and will probably have an active community from launch.
I have not tried godot yet but it’s something I’m going to get to soon
https://godotengine.org/