Make the active weapon rule from the start and just cut the crap if you'll excuse the expression.

There's no point in making a new weight class which is going to end up exactly the same as the featherweights, ie you either build a box or a bot with an incredibly powerful weapon to get rid of boxes. Throw in the active weapon rule and if you don't want to put a weapon on your machine then don't build one for this weight class, simple. I don't really see an active weapon rule putting anyone out of shape if the weight class is only getting off the ground and if you want to build a box rammer then build a featherweight or antweight.

I do like the idea of an adjustable terrain to add another dimension to the fighting. If you wanna build a wedge in this instance feel free, it just might get caught up.