This is a tough call and you will get plenty of advice and suggestions.
I think that the best strategy would be:
-use your DA18-55mm
-check your photographs and assess the most common focal lengths that you use.
-plan your lens roadmap
-have a roadmap.
Among your future choice, you may have to consider a variety of points. Do you need longer focal lengths (than 55 mm)? do you prefer zoom or prime lenses? How much are you prepare to spend? Do you value image quality (IQ) over best-value-for-money? Will you fall for LBA (lens buying addiction)?
If I look back at my own initial investment in lenses, my first purchase was an all-around zoom lens (DA18-250mm) and my second was a fast prime lens (Voigtlander Nokton 58mm f1.4). My third was a FA Limited lens (FA31mm f1.8 Ltd).... Generally, Pentax offers a range of outstanding prime lenses for focal lenses up to 100 mm and these primes (FA Ltd series, DA td series, DA* series) are very highly regarded across all camera brands.
Start to shoot with your gears and current lens, and take your time...