In praise of primes - which next?

Like you, my first prime on Pentax digital was the DA21 - having bought my K10d initially with a DA16-45.

For my own case, the 21 + 70 has been a really nice minimalist combination especially for travelling. The 70 is better than the 21 (unsurprisingly) and sufficiently different to the 21 to really produce a separate kind of picture. However the 21 suits my personal pictorial "eye" much better and I take many more shots with that. I have since put the 35 Ltd macro in between - lovely results, still smallish in general terms but a little larger and heavier compared to the pancake trio. Field of view is rather "vanilla" and obvious, which is sometimes just what you want, sometimes exactly what you don't want. I expect the same would apply to the 40, though maybe less so; in retrospect, I should have trialled that against the 35.

Wideangle scenes can often be covered well by pano stitching the 21 - which I find well-suited to this approach. If planning more than a small percentage of very wide shots, though, that is too restrictive practically speaking. I use a Sigma 10-20 there, but it's far from discreet or pocketable. I've wondered about the DA15, but worry that its rather distinctive presentation of a scene may be a bit of a rather expensive one-trick pony. I've seen some cunning and varied use of it here, though, which does leave the subject to dominate the presentation - so it clearly is possible, assuming your visual imagination tends in that direction.

Don't discount cheap choices - I get a lot of fun and IMO nice results from a manual-focus A/28 which I've owned for 30 years. I also recently picked up a flawless M/135 for £26 which is remarkable, if very slightly more laborious to use.

I used to suggest taping a zoom lens to fix one focal length, so as to "try out" the field-of-view of a prime you are considering. But.. it's not quite the same thing, somehow, subjectively and this can be deceiving. Using a prime rather than a zoom is almost entirely about subjectivity!