I've been using Godot lately, and while it has many flaws (an over-reliance on strings as identifiers, for example), I think the fundamental design is sound and easy to work with: everything is a node in a scene tree.

The engine code, while not up to modern C++ standards, can be pretty easily understood, modified, and then fully recompiled in just a few minutes. It's not suitable for a AAA 3D game, but it's the first engine I've found for 2D C++ development which actually helps me more than it gets in my way.