This is what the basic kit looks like. The main bag is in my new Camelbak Mule 3L. To the right is a good pair of walking shoes, to the left is a Columbia fleece jacket and a good Camo shell jacket to go over. I also made sure to bring good clothes, not pictured. Clothes will vary a lot depending on what area you're going to, that's why I didn't put much emphasis on them here. Also, one would hope that you bring appropriate clothing to begin with, whether or not you're building a kit.
Hat, gloves, two jackets, neck gator, pack above, Nalgene bottle...
Stainless steel bottle cup, extra pack strap, water purification drops, flashlight, extra batteries, Buck folder, Gerber multi-tool, 40ft of cordage, first aid kit (a simple one: neosporin, alcohol wipes, ibuprofin, asprin, antihistamines, gauze, bandages, good tape), duct tape x2 (without the tube they're the size of a AA battery), chapstick x2 (for lips and fire starting), emergency blanket, waterproof matches, Camelbak Tabs (electrolytes), flint and steel. I didn't have a poncho, I need to add one or a small tarp for shelter building. Also not shown, extra socks/underware, other clothes.
One of my discoveries that led to making this kit was the cool folding flint, steel, and magnesium fire starter I found at a local hiking store. It folds and fits into a small pouch, it was fairly expensive ($30), but is very nice quality from a company in Alaska called Kodiak. Folded and in the pouch it's not too big, about the size of my thumb.
One of my biggest concerns with the Airline kit was how to package it and keep the different items from being damaged in my checked bags and not taking up too much space. The Nalgene bottle was a really good idea, nearly all of the smaller items fit inside the bottle, with the cup fitting on the bottom. The Camelbak tabs, first aid kit, knives and multi-tool were the only small items that didn't fit inside. Then, I used my neck gator as packing at the top to keep everything from rattling around.
With the 3L bladder collapsed, and with the added bottle, that gives me an easily packed 4 liters of capacity. The clothes, shoes, and jackets I was already packing, so that's nothing extra. I used the Camelbak Mule pack as a carry on along with my laptop bag, so overall this kit added very little to my pack out but added a lot of capability.
For those of you with travel plans, I hope this gives you some ideas. Thoughts or suggestions always welcome, as usual.