Wait to buy? I'd like to pick up a new camera but I can't make up my mind what I need/want. I'm leaning D500 but if there's a replacement for the D750 in the works, is it worth waiting for?

I know it's apples to oranges, DX, FX, and even the A7III is a compelling option. I can't quite make up my mind which system I want to invest in.

Is there a pressing need for your acquisition before, say, July 2019? If not, then waiting for details on the new releases we've heard about (from Canon, Nikon and Fuji) will allow a much more informed decision. I expect those details will come ~Aug 2018 (4-6 weeks from now) and the new systems from Canon and Nikon will be available in about a year.

If you wait for the next best thing, you'll always be waiting. If you think you're good enough to wait for the next offerings from the major players, then wait. If you don't know what you want/need, I would suggest picking up a D7200 or similar and using it everywhere you go and develop your skills to know what you want/need.

I already have what is the perfect camera for me right now,.... but I am awful interested in the Z mount Nikons that should be coming out soon.

Yep, all companies have products in the pipeline that even bigger, better, & faster. Personally I ordered a D7500 to replace my D7100 because I frequently take photos in limited to dim light, and sometimes the shutter refuses to release because it is hunting for a focus. The D7500 has faster focusing and way better low-light capabilities.

True, I have to give up the D7200's dual SD-slots, 24MP pictures, and external grip battery; but to me it worth it because I never use the 2nd SD slot and I never had a SD card failure where I lost all my photos. The slight sacrifice in 20MP is not that bad because I have only  made one 8x10" photo in my life and that was on my 10MP D40x.

What do you take photos of the most, or what to take photos of most? Same with glass, so you have glass to push the camera that far, etc. That’s something to consider as well IMO.

If you have all DX glass currently, going to a full frame would be really expensive because you’ll lock the camera in DX mode.

I like the selection of FX lenses versus the DX lineup. I think I'm going to try out one of the 16-80/2.8-4.

If I'm going to spend the money on a 17-55/2.8 DX lens, it makes sense to me to spend a little bit more on the FX 24-70/2.8, other than the weird range on a crop sensor. So that's my thoughts with waiting to see what's up with their full frame mirrorless cameras...

The mirrorless are going to be a different lens mount. They’ll have an adapter for F mount lenses to Z mount, but no idea if it’ll ship with the cameras or not. They won’t be reverse compatible 99% likely as well. Upside is they are up to f/0.9 lenses so they’ll be insane in low light.

I'll be ery curious to see some real-world results on z-mount lenses on f-mount bodies with the adaptor.  Nikon has traditionally taken great poaoins to ensure backwards compatibility, let's see how they do with this.

I suspect the adapter is only to allow mounting F-lenses on a Z-mount body (rather than Z-lenses on F-bodies).

Or have you seen a rumor I missed?  :)

That’s all I’ve seen. The mount distance is shorter than an F mount, so the adapter is a spacer and AF passthrough chipset basically. Stretching out the mount would cause AF issues unless there is a corrective lens. Sure it could be done, but image quality would suffer and lenses would get slower. So not much of a point IMO.

OP, Nikon is announcing a new, mirrorless camera that accepts older Nikon lenses. It's on the Nikon USA web site home page.

To OP...first, you have to decide what you want the camera to do. Do you like landscapes? Big sky shots? Choose FX.
 Action and low light? D5. A D850 is a superb choice for a do everything FX platform as well.
Do you like bird pics? Action? Low light? A D500 is great for that.

I only shoot DX. I like buying and using FX lenses because I end up using the sweet spot in the center of the lens.
Vignetting is a non issue too. I shoot sports and low light so the D500 is the final solution for that as well.
I also shoot birds and the narrower field of view of DX improves my apparent focal length 50%...which I prize as well.

Fine art portraiture, landscapes and architectural work is better with a high megapixel FX camera like the D850.

Only you can decide what you will use it for. After you know what you want to do, then you can decide what camera body
you want. Glass is another story. Spend as much as you can and only buy FX. It will outlast whatever body you choose.
The body is like the film...the glass ultimately determines what you can capture. Good glass on a mediocre body is better
than cheap glass on a super-pro body.

I own and use a D500 and can endorse it wholeheartedly. But another body might be much better for you. It just depends
on what you want to do.

The mirrorless is a new mount but will take F mount with an adapter. At least that’s the info I’ve seen.