- Like what you've got: okay, the D200 body the S5 is based upon is still a pretty solid performer. Be aware that digital parts will become a problem at some point, though.
- Dislike what you've got: if you like Fujifilm color, you don't have a lot of choices. The best choice would be to go mirrorless and get an X-Pro2, X-T10, or X-T1, but that means new lenses and flash units, too. If you want to stick with the F-mount you have to move to a Nikon DSLR or buy a used Fujifilm DSLR. The only used Fujifilm I'd even consider is the S5 Pro, so you'd be replacing what you disliked with something you dislike ;~). Seriously, look at the mirrorless Fujifilm cameras, and if they don't do it for you, you'll have to figure out another DSLR brand to switch to, typically Nikon if you want to use the lenses you've acquired.
I keep having people argue with me that the S5 Pro is a 12mp camera. It is not. It's a 6mp camera that has a built-in HDR arrangement, using a compact camera pixel coupled with a DSLR pixel to build extended highlight information. To get to 12mp it uses a unique up-rez routine that makes it perform about like an 8 to 10mp Bayer camera. It is a very unique camera, and nothing has matched it, though we now have cameras with almost as much overall dynamic range without the post processing glitches of the Fujifilm scheme.
But here's the thing: Fujifilm abandoned three different unique sensors in their DSLR line. This is causing multiple issues (and their mirrorless cameras using yet another unique sensor design isn't helping), as software developers simply don't want to spend time supporting all these unique-but-soon-abandoned Fujifilm efforts. Fujifilm basically boxed themselves in a corner, and now their users are in that corner with no logical escape route, even from Fujifilm.
When a company does this constant "let's start over" with its user base, it doesn't engender long term loyalty, thus doesn't build a large user base, which also allows them to keep changing things without affecting many users. It's a vicious cycle. Thus, opting for a Fujifilm body (e.g. an S5 Pro or even one of their new mirrorless entries) means you may getting a one generation bang for your buck. I'd say you have to really, really like something about that sensor if you're going to commit to it.