To upgrade your kit, the first thing you need to identify is what is your limiting factor. It's useful to see your kit in 3 groups. (i) Camera; (ii) Lenses; (iii) Sensor.
Of course you can't mix and match sensors and cameras on DSLRs, as you can do with lenses, but it's useful to see it this way because the same sensor is often used on different cameras and there is no point upgrading your camera to another one that uses the same sensor if that is the limiting factor.
The camera(i) features are things like max shutter speed, flash sync speed, burst speed, photometers, viewfinder coverage, responsiveness, ergonomics. That is, most aspects related to the mechanics of framing, composing, metering and, of course, firing.
The lens(ii) primary feature is, of course, the quality of its optics: the sharpness of its focus, the quality of the colours, the depth of field, the quality of the bokeh and so on. But also, they are largely responsible for the responsiveness and speed of the autofocus.
As noted by @mattdm on the notes, while cameras are somewhat similar in what they try to accomplish, lenses have a wide variety of specifications and they serve different purposes. You may need to add (as opposed to replace) lenses to suit your needs. Maybe a a longer telephoto to photograph wildlife, or a wider angled lens to capture more panoramic landscapes, or whatever you feel you need that your lens doesn't provide.
If you have experience with film photography, you can relate to think of the sensor(iii) separately. Once the image has been composed, the sensor will retain the image. The features you are looking on a sensor are low noise, high sensitivity, good colour reproduction, number of pixels (resolution), etc.
Again, you can't change only the camera or the sensor, but finding out what is the limiting factor is key to understand what camera to switch to, if you find is not the lens that's holding you back.
Try to explore what is it that you miss on the photos you are taking and, before you buy new kit, make sure that the equipment you have is not able to do it. Sometimes the camera is capable but you haven't found the way to make it work.