I don't have any specific suggestions for you but am wondering if you will be replacing any existing lenses or looking to supplement with something longer than 200mm?
Typically portraits (head & shoulders) tend to be between 85mm and 135mm on FX. Since you have DX, your 50mm f/1.8 should be fine for those. If you are looking to do full body shots, some people go wider while others go longer. It really depends on how much room you have between you and your subject. If you go wider, be sure to keep body parts away from the edges of your frame. There is some perspective distortion that occurs which can stretch body parts.
For nature it all depends - if you are specifically looking for wildlife, then definitely something longer. The previous suggestions of 150mm-600mm would be ideal. If you are including flowers in with your definition of nature, then you'd want something shorter. And what you've got is okay for shorter. Nick mentioned a macro lens. A few people here have a Tamron 90mm f/2.8 macro lens which gets good reviews. It could possibly double as a portrait lens if you have enough working room.
If you are looking to replace your 2 current zooms, Nick's suggestion of a 70-300mm lens is good. Typically FX glass is better than basic DX kit lenses it so would be an improvement over your existing 50-200mm lens. Nikon makes a 16mm-80mm f/4 DX lens that is the kit lens for the D500. It is the DX equivalent of the Nikon 24-120mm f/4 that is the kit for some of the FX bodies. But Dan's suggestion of the Tamron 28-75mm wouldn't be as expensive - and I believe some other members have that lens, too. You might want to do a forum search to see some images.
Good luck with whatever you choose!