- Like what you've got: Stick with it.
- Dislike what you've got: Consider upgrading. What are you thinking? These were seminal cameras that are still highly competitive today, particularly the D3s. However, the D6 would be a strong step upwards in terms of performance for focus and frames per second. Not a big change in dynamic range at high ISO, though you should get more detail/acuity at the same print size. The one thing that you should consider is that Nikon will likely move to mirrorless in this line in the near future (e.g. 2020/21).
- Upgrade to mirrorless: Z8 or Z9.