Currently:

SubScene {
 Parent "{GUID}worlds/world.ent"
}

Proposed:

SubScene {
 Parents {
  "{GUID}worlds/world.ent",
  "{GUID}worlds/world2.ent"
 }
}

While this feature isn't essential, it could be useful for more complex inheritance setups. In a way, it can be thought of as an "implements" instead of "extends".
Just as an example, consider this:

• BaseWorld, Parent: ""
• PreWarWorld, Parent: "BaseWorld"
• PostWarWorld, Parent: "BaseWorld"

Without this feature, you would create two parallel independent GM modes:

• GM_PreWar, Parent: "PreWarWorld"
• GM_PostWar, Parent: "PostWarWorld"

With this feature, you could add:

• GM_Common, Parents: ""
• GM_PreWar, Parents: "PreWarWorld", "GM_Common"
• GM_PreWar, Parents: "PostWarWorld", "GM_Common"

While it adds one more world here, the effort would still be lower since you only have to configure and maintain one of them.

I know that this is currently achievable using prefabs, but for more complex things, it would be more convenient to be able to do the same thing with entire worlds.
As I said, this isn't essential by any means, but should not be that hard to do in case you revisit this at some point.