I'm not sure if this would be easy or even possible, but i've always wanted a symmetrical building mode.
So I choose where the line of symmetry would be and all the bricks I place on one side go on the other, in the same place.
Either you want a super glitchy client mod, or a less, but still glitchy server mod.
Client sided symmetry would require shifting bricks across the line of symmetry and back,
Server sided could either be placing 2 bricks at once, or moving 2 ghost bricks, like the duplorcator
