Is it the 2wd or 4x4 Slash?
If it is the 4x4 you will notice that it gets quickly filled with dirt and gravel, it can pick up LOTS of gravel in no time. I recommend adding some inner fenders like these:
https://scorchedparts.co.uk/products/traxxas-slash-4x4-lcg-inner-fenders I have them on my Slash and it works great.
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) .