I own more lenses than I do cameras. I'm betting you probably do to. Here are some things to think about when buying lenses.
The image that comes out of your camera is recorded on the sensor but formed by the lens. The recorded image can be no better than what is projected on it by the lens in use at the time. A good lens attached to an average camera may be limited by the resolution and capabilities of that camera, but a top of the line camera is equally limited when used with a less than perfect lens. Both components matter.
Even as I upgrade my camera periodically, I still own many quite a few lenses I bought years and years ago. Camera technology today changes faster than lens technology. Think of buying a new lens as an investment. If well cared for, it could last you for many years. Buy the best lenses you can afford. If you have a budget for upgrading all your gear, you will generally be better off in the long run by prioritizing your spending more to lenses than camera.
A bad lens won't get any better if you upgrade to a higher resolution body. You may not notice its deficiencies on your old camera, but on a good enough body you will. There are few feelings worse than spending your savings on a new camera only to find you'll have to replace your lenses as well to make the most of it. Your best insurance against this situation is to buy good lenses to begin with. In the long run, doing so can actually save you money.
For those of us in the Nikon camp, we've come to expect that the Nikon F-mount is ubiquitous and that we can mount any Nikon compatible lens on any Nikon body, more or less. Now that we are at the dawn of the mirrorless camera era and both Nikon and Canon have introduced new, larger lens mounts, it would seem likely that Nikon's new Z-mount will replace the F-mount over time as the trend towards mirrorless continues. Thankfully, the nifty FTZ adapter allows Nikon users to maintain compatibility with new bodies, and I expect that Canon will have something similar for their users. Those of you who have made the switch to mirrorless can rest assured that Nikon is doing their best to maintain backward compatibility. It seems reasonable to expect that the F-mount will remain a viable option for some time to come.
Historically, third-party lenses have often been less expensive than Nikon and Canon's equivalents. But OEM lenses generally feature better build quality and retain a higher percentage of their price for resale than Tamon, Sigma and the others. I've owned some Nikon lenses that actually appreciated in value over time because they have such good reputations. Nikon's own line of lenses though isn't as extensive as is Canon's, and in such cases, third-party makers clearly rule. Thankfully, the internet gives us access to numerous lens review sites that can help separate the good ones from the merely mediocre. Most any Nikon lens I know will be good, but I always thoroughly research third-party lenses before buying.
There is less need these days to screw filters on the front of your lenses since so much can now be done post-capture in Lightroom and Photoshop. But a protective filter for trips to the beach or a polarizer to remove glare are still warranted as are others when the need arises. Given this, it's a good idea to think about filter size when buying lenses. Nikon users have long had the "holy trinity" of f/2.8 lenses with 77mm threads to lust over, even as the exact lenses that make up that trinity changes over time. It's great to be able to screw the same filter on any lens without the need to fiddle with adapters.
There once was a time when prime lenses tended to be better than zooms, but that really isn't true anymore. Extreme zooms that let you never have to change lenses can be convenient, but they remain the exception in terms of quality. It's just not really possible to optimize the optics over such a broad range of focal lengths. But "professional" zooms with more modest zoom ranges are every bit as good as primes, and indeed some of the sharpest lenses available today are zooms. Having said this, if you really only need (or want) a single focal length, a prime will almost certainly be both cheaper and lighter weight than an equivalent aperture zoom that covers that focal length. But that same zoom will in turn be cheaper and lighter than buying (and carrying around) a whole camera bag of primes to cover that range. In the end, which type you buy should be dictated on your needs. Myself, I appreciate zoom lenses since it isn't really always possible to step forward or back when shooting in nature. I try not to stand in flowing stream beds or step backward over a cliff.
Larger apertures are helpful both to create images with shallow depth of field and to improve viewfinder brightness. The lens doesn't stop down to the selected shooting aperture until the shutter release is pressed. When composing, the lens is fully open to its maximum aperture. It's your choice what that aperture will be. Low light shooting isn't as dependent on wide apertures as it once was since compensating with a higher ISO isn't as scary as it was on earlier digital cameras. It used to be almost unthinkable to boost the ISO. Thankfully, those days are behind us.
Image Stabilization has become fairly common, but you don't really need it on wide angle lenses so save your money. There's a rule of thumb that says you can safely hand-hold a camera at shutter speeds down to one over the focal length. That means you wouldn't need Image Stabilization on a 30mm lens unless shooting slower than 1/30 second. And if you are, you should have a tripod anyway.
Consider that you may not need to buy a new lens at all. It may seem that way but be sure you have thought it through and that your current lenses really are a limiting factor. It can be temping to blame your equipment for any images that don't come out as you hoped, but that isn't always the case. Don't forget your own role in the process. Push yourself to get the most out of your lenses, new or old.