I am afraid everyone who posts will try to impose their ideas on you, so at the end of this I will link to a posting of how to build a lens system from scratch.
Generally, my approach is to extend range and capablities before upgrading a specific lens.
As a result, given the ultra zoom you have, I woudl suggest an ultra wide,
Whether it is a 10-20 sigma, or 10-17 pentax is up to you and your tastes, I thing personally the fisheye is a little limited in use compared to the 10-20.
I have a 10-20 and it takes about 20% of all my photos, with 60% of the photos taken on it at 10mm. WHen I travel, it takes 50-70% of all shots.
now for building a kit from scratch,
https://www.pentaxforums.com/forums/407974-post10.html