Since my game will go in the direction of 4X (explore, expand, exploit, exterminate), it will become very menu-heavy.
In the Assetstore you can already find some things in that direction that should help you to create a GUI.
For me the question is whether things like NGUI or DoozyUI are really useful additions or replacements for the Unity UI system.
In my case, I am using DoozyUI , as far as I am using it , it helps a lot with animating UI, having a consistent flow of UI and if you want to make a 3D UI ( world canvas) and animate things like camera and that kind of stuff it helps. It have some downs ( like learning it, some improvement down and that kind of stuff), with NGUI you will have to build it on your own but it will be more powerful and flexible