Would it be useful to have a piston which can extend 2 blocks instead of 1 block?
I'm thinking that when a signal is applied, the arm extends 1 block taking the same time as a normal piston and then extends another 1 block again in the same time as a normal piston, so the overall time taken is twice as long as for a normal piston. it would be a "clunk-clunk" two step process each way, extending and retracting.
I'd think that this would make it easier to construct drawbridges, for one thing. Probably also elevators. There would be less need for redstone circuitry and tricky timing of signals.
Crafting would be one sticky piston on the bottom, an iron bar in the middle and either an ordinary or a sticky piston at the top. The result would be either an ordinary or a sticky double-extender piston.
The double-extender piston would behave just like an ordinary piston on each extension or retraction. If the piston could only extend one block (by meeting the block push limit or by hitting obsidian), it would simply stop and subsequently only retract one block.