This is kind of yet another "which engine" post (so recommendations welcome), but not quite.
I'm particularly interested in hearing what people have to say about productivity of modern commercial engines (ex. Unreal 4) vs that of popular open source engines for a single developer. That alone may be enough to rule out most or all commercial stuff, but maybe I'm wrong!
I've always assumed that a commercial engine would require a ton of "bootstrapping", have a huge learning curve, would assume your requirements are all AAA (thus making my more trivial requirements actually harder, vs a simpler engine designed for the little guys), and that it'd be completely infeasible for a single dev to ever use. But I've decided to re-visit that assumption.
A few constraints to be aware of:
-Not particularly interested in toolset-oriented engines (ex. Unity?)
-C++ should be a first class citizen - I don't want to spend my time learning tools, I'd rather spend my time learning a well documented, high quality API where I can stay in code world as much as possible.
-"Tools" like static scene or level builders are okay I guess, but probably not very helpful since I'm interested in space-sim type projects centered around travel, "building" your own scenes/bases, and some procedural content.
-Linux support (at least as a runtime platform)
Edit:
I should probably state that by "commercial" I mean proprietary, AAA capable, corporate backed, and so on. I guess that commercial/noncomm line is starting to blur a bit lately...