Do you mean first you would create your campaign then when your finished you can gm it with friends but can't edit the quest terrain etc only spawn in and posses monsters? Please elaborate
Well, yes. When I look at it though, there's actually 3 layers to this.
1. There's classic modding. This is like what you have in Skyrim or XCOM 2. In Skyrim, you find plenty of mods that improve the graphics. Also, in XCOM 2, if I want to create a new alien to play with, then this is added to the core game and any existing save games are affected. Ideally, all content-creation should be pushed to the following layers.
2. There are creation tools. This would be a bit like RPGMaker or Battle for Wesnoth. You can build-your-own campaign/module that can be shared and played without having to modify the core campaign. I think we can expect this to include creating custom creatures/equipment with custom assets. This will give you essentially full control over your own custom campaign, but you would need to create it in advance.
3. GM mode allows you to manage a gameplay session mostly in real time.
You will definitely be able to do everything you want to do. It's just not entirely clear which features will exist in which layer. For example, it's not yet clear whether you'll be able to do things like control dialogue in the GM mode or if you'll be forced to plan 100% of your dialogue in advance in the creation tools. It's also not clear how easy the creation tools will be to use for novices like myself.
But I understand it's norD's job to read your suggestions.