This is actually a tricky question, since there are so many different purposes for which I might dream of a new lens.
- Best (most dreamful) of what I already have: DA* 60-250 (my only * lens so far and the most expensive too)
- What I would like to have next (of existing models): DFA 150-450 ... for longer reach even w/o teleconverter, ... OR perhaps HD FA 1,8/31 Limited just for its fame
- What I wish Pentax (Ricoh) to release some day: DFA 18 with max aperture 2,0 or larger. (Thus eagerly waiting for further information of the forthcoming DFA 21...)
Another dream is, that 3rd party manufacturers (like Sigma) return to increase their supply of modern K-mount lenses, so there might be something to dream of also beyond Pentax brand lenses.
And all this applies to today only. Tomorrow my dreams may already be slightly different!