I just finished designing an entire asset management pipeline and christ on a fucking pogo stick, if it isn't convoluted.

Theres a lot of game engines out there, but all of them do it a little different. They all tackle a slightly different problem, without even realizing it.

1. asset management

2. asset change management

3. behavior change management

4. data management

5. combinatorial design management.

6. Combinatorial Behavior management

7. Feature completion

ASSET MANAGEMENT is exactly what it says on the tin.

ASSET CHANGE management can be thought of handling the import, export, formatting, platform specific packing, and versioning (including forking) of an asset.

BEHAVIORAL CHANGE management is a subset of asset management, because code is a subset of assets (depending on how you define 'assets'). The oldest known example of this is commenting and uncommenting code.
Or worse, printf debugging.

This can be file versioning, basic undo services, graph management of forks and mergers, toggles for features or modules, etc.

DATA management is about anything that doesn't fall into the other categories, everything from mission text to npc dialogues, quests, location names, item stats, the works. Anything you'd be tempted to put in a database, falls under this category. Haven't yet seen many engines offer this as an explicit built in tool as of yet, because the other problems are non-trivial as is, so this is a bit of low hanging fruit that gets handled by external tools, or loaded from formats as simple as json.

COMBINATORIAL DESIGN management is the idea of prefabbing, blueprints of broader object design using nested prototypes of existing game objects, to create more complex, reusable set pieces. Unity did this well. GM does this in part.

COMBINATORIAL BEHAVIOR management is entity-component systems, plus tooling to make it easy to add, remove, and configure components and their values on entity blueprints, also not uncommon. Both stencyl and unity do this. GM has a precursor to this in the form of configurable fields, but these fields are not based on component scripts attached to objects.

FEATURE COMPLETION is that set of gameplay mechanics or styles of design that an engine naturally makes easier to include or build in a game.

I don't think I'm aiming for all that, but I think at minimum a good engine has to do asset management, behavioral change management, prefabs, and entity-component systems with management tools for that. And ideally, asset change management.