If your Slash has the plastic Ultra Shocks, I recommend replacing the plastic shock caps with some in Aluminium, if you don't do that the shock cap WILL pop off at some point. You can just re-fill oil and put it back on, but the thread on the shocks gets a little more damaged each time and the cap comes off more and more often. The alu caps fixes the problems, but you need to use it before the thread is too damaged. If you have the GTR shocks there are no problems with shock caps.
Besides that I wouldn't replace anything before it breaks. The Slash is an awesome car and don't need updated besides the two mentioned above.
My Slash has a LOT of upgrades, for instance it has lots of RPM parts and all electronics has been replaced with non-Traxxas. But nothing was replaces before the original part broke. When something plastic breaks consider using RPM parts instead, when electronics breaks consider using something better (Hobbywing Max10 SCT motor/esc combo is cheaper than Traxxas and much better) .
There are a ton of upgrades for the slash check out this link: https://www.rcsuperstore.com/cars-trucks-parts/traxxas/1-10-rally/?sort=featured&page=11
I was looking up rally parts but there is a lot for the slash there. I have a decently modded one running 3S and 4S and they can jump clean off the track I drive them on . 3s is a lot of fun 4s you start stressing driveline pieces so if an insane amount of power is what your looking for id upgrade to X-01 diffs, tekno RC center drive shaft, good quality CVD's and a center diff with aluminum diff housing. Also some GOOD wheels. My metal hex nuts started stripping the inside of the wheel hex on 3s lol.