The UI should be 100% controllable without a mouse, like in ADoM. That much is a given, and should be implemented as early as possible.

At the same time, considering the advantages of the JADE platform, you could theoretically allow using mouse completely without a keyboard either, similarly to how you would do it on a touchscreen device. Using mouse cursor to walk would help especially for playing on laptops without separate numpad, where diagonal movement/aiming is a considerable hassle.