The way I see the only way GM mode ever becoming popular is it has to be simple and intuitive. While being flexible when necessary.

I imagine DM mode divided into two separate operating modes.

Preparation Mode:
Live Mode:

The Preparation mode is essentially a simplified version of the editor. It lets you build the map, create factions, and faction associations ( which groups are allies, which groups are enemies, etc ) It also lets you create two types of units. Profile based units, which creates units randomly based on a set of parameters. Unique units which look exactly as set by the creator.

The 2nd part of preparation mode are premade scripts. Something like a merchant script - soldier script, archer script etc. Essentially Ai packages which can either be from Larian or custom made. So when you place a unit in live mode, you can just assign the package to the unit and they will act accordingly based on that package.

Finally when you finish "preparing" you save it as a package file and the package file is loaded when you host and gets "distributed" to the clients. This package contains the map \ unique scripts \ and other custom assets.

When the game is in motion you have "Live Mode"
Live mode would essentially be like a control station of sorts.

Lighting \ Weather Button - This changes the light to dark \ dusk \ noon \ night. Which lets you set the mood for the area.

An object button - which lets you drop objects. Clicking on the object lets you move it around and rotate it. Right clicking on the object lets you set the object as a container \ trap \ etc. Lets you set the weight of the object as well as health and description.

Then an NPC button which lets you choose from the prefabricated characters you made in preperation mode, either a unique character or profile based character.

Finally by clicking on the character you can order it to do things such as; sit, move, run, interact with "x" object. Or if you have a custom script, it will run that.

Also there are 2 types of dialogs.
1. Pre fabricated dialogs created in preperation mode. For example gate gaurd etc.

2. Live Dialogs which opens a some what of a chat window with the avatar [ 3d ] of the character appearing next to each statement the character makes. Where the DM can also activate a manual Dice Roll or rock paper scissors match with player.

I think by keeping Live mode simple - it will allow much flexibility for more users. While making sure the DM's are constantly being engaged. Especially since you forcibly split preparation mode to an offline event where the DM has to first prepare everything in advance, instead of forcing players to wait for the DM to set the stage.