An interesting idea for a mod would be to make it so that similar to terraria, where the inventory is in the top left, and if you click a block (inside the inventory, to move it to another inventory slot), then you can click the ground, and it would place that block. This would mean that you can place blocks from inside the inventory and not have to select the block inside the hotbar slot. This would be especially useful for building.
Also, it would be useful to be able to move while inside of the inventory.
I do not understand how modding works, so I do not know if this is even possible, but if it is, that mod would be amazing, and would probably be used by a lot of builders.