I don't think that would be my approach. that sort of thing should probably be provided by add-ons. they could definitely be add-ons you make that are included with the game by default, and I think that would actually provide additional benefits, like giving you an opportunity to actually work with whatever interface it is that you wind up making for extending the game, to see whether it really is practical, and useful, instead of just looking at it from the outside perspective of the designer
also just the idea of including something so opinionated as an inherent part of a sandbox game like this seems a little........ not ideal
I can release it as an external add-on if that would be best. But yeah, I would like to have some type of RPG system players can use right away, once the game is released. It would also provide an easy reference for those who want to make their own later on as well.
Will it be easily fully modable like Blockland/Roblox?
We're planning on making modding as easy as possible (with plenty of resources and reference material for content creators). It should basically be the same as Blockland's level of modability, and might also include support for changing brick textures, GUI graphics, etc. We haven't gotten that far yet, so we'll go into detail regarding that later on.
What I'm thinking for the inventory is having the item hotbar as usual (in the bottom-center of the screen). When you have a "brick tool" in one of your hands, a second "brick" inventory hotbar will appear above that, allowing you to wield a gun and build at the same time. It also shouldn't occupy too much of the screen; if necessary the GUIs can easily be rescaled. Also, if you have a backpack, the additonal inventory hotbars will appear above the items bar, with the amount of slot columns and rows determined by the item's datablock. When you're viewing a storage brick's or vehicle's inventory, that inventory window will show up in the center or top-center of the screen, which will minimize the amount of windows and GUIs the player has to manage. All of the inventories will most likely be adjustable in size as well.
I'm working on a mockup example, and will post it soon.