I remember when we all started to see the first inages of the DA560, and when I saw the pictures, it kind of looked collapsible in design, I was kind of hoping it was a 400/f4 when collasped and a 560/f5.6 when extended. That would have been something special. Maybe, they could make a FF version that did either that, so you have two lenses in one, or a 400/f4, with a built in 1.4x converter, similar to the Canon lenses.
OR.....Pentax could do something designed similarly to the Apo-Telyt-R Module System? Make the Big front element and differnt modules for different lenses lengths, Make a base model 300/f2.8, and have the option of going to a 400/f4 or 560/f5.6 by buying the extra modules,