Ask yourself:
What do I want to do that I can't do with what I have?
One way to look at it is,
coverage. Consider that the 'normal' focal length for your camera is 30mm. You have focal lengths from 18-200mm covered -- that's fairly wide to pretty long. You can shoot whatever you like in that range. Then you can run a program that looks at the EXIF data from your images, and plots which focal lengths you actually used. If you see clusters around certain numbers, THOSE are where you should think of buying prime lenses.
Another viewpoint:
lens speed. Your zooms are not especially 'fast' -- their maximum apertures aren't real large. There are good reasons to have one or more lenses that open to f/2 or f/1.7 or f/1.4. You can shoot faster-moving subjects, and/or in less light, and/or with artistic effects. I like prime lenses that are considerably faster than my zooms at critical focal lengths.
Note on
aperture: An f-stop is a fraction, the ratio of the maximum opening to the focal length. So f/2 is a larger aperture than f/4, just as 1/2 is a bigger number than 1/4.
Another consideration:
shoot what? If you want to shoot close-up or macro subjects, or more-or-less distant moving subjects, or subjects in cramped spaces, or perfectly undistorted highly-detailed 'scapes, specialized lenses are available. I bought my first dSLR specifically because of the lenses available, that could work in my areas of interest: ultrawide, quite long, low light, and macro.
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I buy and sell used lenses online. I'm retired with no discretionary income, so to buy stuff, I must first sell stuff. So I buy odd lots of photo gear, pick out goodies, sell the rest bit-by-bit, and make enough profit to buy more lenses. I'm up to around 200 in under 3 years, heh heh. (That number will drop by a dozen or two as I sell off the next batch.) What I keep are mostly old manual primes. My average cost: under US$20 each. As my time is worthless, I can spend much of it searching for deals, and I've been able to buy many fine lenses for a fraction of their going prices. All that's needed are patience, research, experience, and low risk tolerance.
This has put many lenses into my hands to try out, to see if I like them. And I've put together a basic kit, some which I'm most likely to have in my carry bag on any day: 3 AF zooms and a few MF (manual focus) primes. I'll select from some (not all at once!!) of these:
Zooms: 10-17 fisheye, 10-24 ultrawide, 18-250 superzoom -- all were new, not cheap
Primes: 16/2.8, 24/2, 28/2, 35/2, 50/1.4, 58/2. 85/2, 135/2.5 -- mostly used and cheap
Specials: a macro setup; a 500mm tele; some with 'character' -- again, used and cheap
This kit matches my personal style, with glass I've found useful for specific needs. Your kit may become similar, or quite different, depending on what and how you shoot. Much of my kit is opportunist -- I found a good deal in a common (or weird) niche and grabbed it, rather than planning for some specific use. Some people map-out a purchase path --
GOTTA HAVE THE 15+31+55+77+100, ON SCHEDULE! I just try everything and see what works. I tell myself I'll sell the rest. Eventually. Right.
Anyway, those are some ideas about lens selection. Hay, it's only money. Good luck!