This is always a bit hard to figure for others, but here is what I did for my small kit. I found a short plastic vial (thin plastic) about the size of a 123A battery. I put several hooks of each size ranging from size 16 to 4, several split shot lead weights, a couple of swivels and 3 or 4 flies....specifically Hare's Ears, and Stimulators. Around here Hare's Ears are a great general nymph pattern and Stimulators are a good general dry fly with hopper/stone/caddis looks. I carry a braided tapered leader about 10' long, a good bit of 8 or 10 lb monofilament and about 10m of braided nylon line. The nylon cord that I carry is heavy enough to be fashioned into a trot line using droppers of the monofilament which are then tied to the hooks.

I also have an 8/0 hook for a gaff.

The tapered leader and a fly or a baited hook, and possiblilities for a trot line are covered...all for a tiny amount of space and very inexpensively.

I think the flies are an overlooked bit that can really add value to a kit, but your location and stream types are really the big factor for these as is a bit of fishing skill or knowledge. Big water without a rod and fly line these are going to be harder to use.