I don't know how much you plan to spend, and how many lenses you want, but here are a few suggestions:
- The 15mm limited, great little super wide angle lens than will supplement your 21 perfectly. An used DA 14mm is also as good, but it's bigger
- I also owned the 21 and the 35 limited before, but got tired of constantly switching lenses betwen these 2 standard focal lengths, so I picked up a 20-40 limited zoom, still have it today
- A 70mm limited (or 77 if budget is no issue) to bridge the gap between 35 and 100.
- Plenty of telephoto options to extend that 100mm range either: 50-135, 60-250, or *200 if a prime is preferred.
- Another good option would be getting a versatile standard zoom like 16-85 or 18-135 for main use, and those primes with become its supplements for when you need them.
- Fisheye territory?
- 35mm 2.4 is plenty fast, but personally I find that, occasionally one needs to shoot at very fast aperture (1.4, 1.8) so a 50mm 1.4 -> Sigma 30 -> FA 31 or DA* 55 maybe (depend on budget).
- Buy all current Pentax lenses that are currently being offered on the market, and start to sell later ones that you don't like