I found this video which talks about why you shouldn't upgrade guns. While upgrading a gun can be something you might want to do eventually, he does make some good points.
I agree largely with what he's saying here. Fundamentals drilled to a high degree of proficiency will make your shooting far better than any add on. That has to come first. Once you've done that, you really have to ask yourself what you need your gun to do. If you've taken the time to settle into the fundamentals, you will now have some actual answers to this rather than guesses.
For me, on a rifle, a good sling (the new Vickers type adjustable 2 point) and flashlight are mandatory. This lets you sling the gun and work with hands or effectively transition to handgun with a two handed grip. The flashlight just lets you identify targets. This too is mandatory before pressing the trigger. Everything else is pretty un-needed. After that, a good optic is next. In fact, those three things are the only things I have on my platform.
But again, this comes AFTER you're competent.
As for pistol craft, I'm a light guy as well. You MUST see targets before you press the trigger. I know, it seems to be a midnight shift thing, but there's plenty of houses that you'll want illumination in as well. Please note, I agree, the light won't let you press the trigger cleaner or line the sites up better. But it will paint your target so you know if you need to do those things in the first place.
I see a future where an optic (RDS) on a carbine will be accepted as "stock" just like a scope on a bolt weapon.
Some folks are currently running RDS only with no iron back-ups. Various nations field rifles with no iron sights (Britain led the way there many years ago)..even the US Army is now qualifying soldiers with RDS as the "primary" (vs Irons) these days.
As battery life increases (the Comp M4 has an 8 YEAR constant on life on a single battery) and durability advances I can see an RDS for a combat carbine becoming a standard vs an "upgrade".
I run an Eotech with back-up irons. I was an old dog who qualified with irons so I can't let go...lol!
Well some modifications you might make early on after getting used to your weapon. For instance, for a tactical shotgun a shell holder on the side of the receiver can be a good early edition and it can really help with making port loading faster and more efficient.