I totally agree that the quality of the hooks can make a big difference. I use an assortment of hooks including British Partridge and Japanese Tiemco. I carry standard wire, 1X light wire and 1X heavy wire hooks in a wide assortment of sizes from #16 to 4X. All hooks are inspected and sharpened if needed before inclusion in the kit.

I carry a prebuilt tapered leader with a woven butt as my primary fishing line. This can be tied to heavier monofilament or cord or even a stick as needed. Many kits have very heavy mono which is limiting. My tapered leader terminates with 4X tippet and this could be trimmed back to heavier sections if needed.

Another thing I think that is overlooked in most kits is an assortment of decent flies, I carry some general nymph patterns (Hare's Ear, Muskrat) and a few general surface patterns (Stimulator).