A new part I'd love to see one day is a redesign of the minifigure shoulder to allow it to rotate on a secondary axis. This design uses a simple slot piece in the shoulder (blue) that connects to the rest of the arm (red) with an interior cross pin. With this additional hinge, arms could be posed out wide or hands up on either side of the minifigure's head. More posing options for the lovable minifigure.
I'll tell anybody who listens (and many who won't) that the ultimate LEGO element is the sausage - its versatility of attachment points, with pins at either end, a constant bar-diameter along the length, and an arc that allows for clip attachments at any point. Said arc also allows for its use to create unique off-stud connections and changes in direction. But it has a flaw. A single, terrible flaw.
What, I hear you ask, could be the flaw in this beautiful element?
There's simply not enough of it.
I present to you the solution. The HexaSausage. Six times the amount of sausage for six times the connection potential.* No angle of attachment is left behind. Clip space for days. It's the part you never knew you were missing, but now won't be able to live without.
*Actual connection potential may vary.