I confess that I don't have BESM 3e, :\ . Can't afford it right now (but I do absolutely want it). I just couldn't resist asking...
What I am thinking at the moment is a setting where fantasy mecha and magic meet half-way. A world where you have to be able to use magic to use a mecha, and the stronger your magic the better you can utilize your mechas capabilities, but conversely piloting a powerful mecha makes your magic better as well. The story focuses around the hunt for the legendary McGuffin, the supposed blueprint for the first mecha built hundreds or thousands of years ago, which it is believed can shed light on what makes magic work. The primary villain uncovered an ancient mecha, built off of the original blueprint, and now commands the most powerful magic in the land, though his control of it is not precise, as he was but a weak mage before uncovering his new treaure.