Inventories and other such menus can already be done. Example: https://toxworks.itch.io/menu-system-in-gb Of course, more flexibility is always welcome, e.g. being able to use Actors as items in a GBS menu.
What would a custom actor entail? You can already customize how actors look, move, and behave, and you can customize their animations beyond the default animation types with scripts. The main missing feature is different actor sizes.
You can already Eject Engine and edit the C code directly to add additional features, and subsequent builds from within GBS will use your modified code, AFAIK.
An isometric option would be interesting to see! The main differences would be player movement, which would be at angles rather than directly vertical/horizontal, and some way to mark the heights of tiles in the collision editor. Presumably there would need to be a way to set how far an actor can jump (i.e. whether a given height difference counts as collision or not), but this could probably be controlled with scripts too.
The GB can't do 3D, 3D can only be faked with different backgrounds. If you're talking about first-person games, those can already be created quite easily. Just hide the Player and switch scenes based on the directional inputs. Example: https://jojoruteon.itch.io/3dmaze-gbstudio The ability to animate backgrounds could be used to make these types of games feel more 3D by swapping tiles live, but modifying the VRAM to achieve this is difficult to make fast in the general case. ( I wouldn't be surprised if some basic 3D demos existed on the GB, but doing a whole game in 3D at a playable framerate is beyond the GB's capability, as it just doesn't play well with the way the hardware works.)
I would like the following features to be added:-
Very good engine. It will be a lot useful if the features are implemented. Vote who of you agree with me.