Ah, yes, that's a good idea about having a keyboard button (likely "M") to pop all menu's out. I should figure a way of allowing each to individually pop out, as well as adding an "on touch" auto-popout too.
Regarding the user interface: I think it will all be integrated into the menu system. As I said, the way the skills and the stats menu come from the left and top, the inventory/equip screen will pop out from the right, and the hotbar will pop out from the bottom.
The goal is to keep an area of game play visible, keeping the left and right side menu's semi-transparent so players can see mobs that may have spawned coming for them, or other players that are coming. I'm considering putting all the stats (the top menu) on the right side, and putting the more accessible and less gameplay-intrusive top menu to task as the inventory/equipment screen.
As for the bottom menu, the "hotbar" if you will, it will likely remain up at all times and should be exempt from the "hide all menus" function (but not from the "show all" in case the user personally hides the menu). The attack and jump button should be on the hotbar, but not able to be hidden with the bar, as it is a necessary feature. It should also look different than the rest of the hot bar. (The hotbar will contain skills or spells dragged from the skill list or spell books).
Text notifications will be limited to skill leveling and display it in a similar style as Elder Scrolls V did it, displaying your level progress. (No levels in my game, so it will show how much longer until a skill's governing stat is raised by a point.)
If you have any tips based on the things I've mentioned here, please tell me. I'm not beyond rearranging the menus because it's all on the fly anyways. The game I pretty much have written out and semi-planned as much as a one-man army can, but menu's were defiantly an afterthought.
I appreciate the help you have given, and any you may yet enlighten me on further.