Random thought talking about pieces and stuff: is there something like a two-sided single piece (don't even really know the terms, haha). Like one of the little single stud circle pieces, but both sides go into other pieces instead? I know they exist with a single square piece like perpendicular to them IIRC, but I don't think I've seen them standalone ever. I can't imagine it would be a super useful thing, but I'd think every once in a while it'd be good to be able to attach two pieces facing in the opposite direction.
Just a random thought I had the other night, as it's a "simple" piece in theory, but like I've said I've never seen one.