You should follow up with DA15, DA21, DA35, DA40, DA55, DA200 and DA300
What is the rational for the change in lenses? That should help us have a more objective look at what is a better next step. It will depend on preference as well (eg. don't want to change lens; don't like primes; don't like heavy lenses; large/small budget, etc)
If you just mean that you want a complete coverage from WA to Tele :
Option 1 (more primes)
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1. UWA - DA15, 16-45, Sigma 10-20, DA 12-24
2. 35mm (any from Pentax)
3. 85mm (you already have)
Option 2
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1. UWA - Sigma 10-20, DA 12-24
2. 28-300 (you already have)
85mm for portraits and shallow DOF shots.
There are plenty of other combination as you are already aware.
If you can do most of your shots using a 28-300 while reserving 10-20 for landscapes and ultra wide shots, you can't go wrong with Option#2. You will end up with a UWA anyway (cause the choices are down to 10-20, 12-24 and not much else), so if you ever get to ridding yourself of the 28-300 for other configurations the UWA end of things won't change too much.