We all know you have the one old lens you'd love to see remade in the pentax lineup. For me I have two!
1. I'd love to see the M75-150 refreshed. A DFA 75-150/4 HD DC PLM Limited! This lens as is has some amazing rendering already, with some improved coatings and tweaking to the optics it'd be out of this world.
Some sample shots from this wonderful M lens:
M75-150 f4 | Flickr
2. The original 8 element 50 1.4 tak. I'd love to see this lens with modern coatings and autofocus. While I know this would never happen I'm pretty sure I'd buy this over the DFA 50 1.4