Welcome to Righteous Scraps, a side story I am making because my energy reserves are shot and command and conquer + pikmin + madness is fun.
The rules are basically as simple as said in the OP: You think up an item, a unit, an artifact, a building, whatever it is you want. Detail what it can do (though be a bit conservative, most units have 1 thing they are good at and most items do 1 thing uniquely, you know how RTS games are...)
So yeah, get creative! The limiting factor is how powerful the thing you create is, as it will need more resources to be produced (and maybe special items if the concept is unique enough.)
Even if the item you want cannot be created right now, a "shopping list" of sorts can be created, to consider what weird contraptions you've seen in your minds eye to bring to the war next.
There will be blood.