Unity 2017.2 is now available

Unreal isn't actually oss but source-available so works well enough. Why not try godot and see if it's robust enough for you? Another open-source engine you could look at is urho3d (it still supports 2d games).

For 2d stuff, haxe is pretty good. If you have any Actionscript 3 experience, a lot of memories will come rushing back to you.

I haven't tried Godot yet but it looks really compelling. I sure wish they didn't make up a new language to script with though, there are already so many good choices. :/

I've been following Godot for some time. Development of 3.0 is progressing nicely and looking to have some great features... C#, GDNative (script bindings for other languages: D, Rust, etc.), asset library, a more modern graphics pipeline

My understanding is that those coming from Unity often get tripped up by Godot's scene/instancing system because it's somewhat different than how prefabs work. Subtrees in a scene can be broken out into a separate scene and instanced. Elements as basic as a bullet should be implemented as a scene and then instanced and customized as needed. A common sentiment I've seen is, once it "clicks," they can't imagine going back.

I've only spent a few hours in Godot since recently cloning/building the master branch, and I already have a nice force-directed graph implemented. If it gets to a point where the number of graph nodes affects performance, I can move the force calculations from GDScript to cpp or another native option.

For 2D games, GameMaker is good but has its own scripting language but the language is very similar to javascript.