Well, it seems the major conflict is between knife/multitool and fire-making equipment....

Well, I'd consider buying (if one is available) or else improvising a multitool that includes some sort of firestarting device. Perhaps a small flint could be included as one of the tool blades, or a lighter could somehow be built in along with a small reservoir for lighter fluid. I guess you could call it cheating, but nobody said a multitool counted as more than one item <img src="images/graemlins/smile.gif" alt="" />

You'd still need decent tinder, but you'd have a better chance of improvising that (from scraps of clothing or paper, shaves of dry wood, etc.)

I think the point is, that the more crucial the ability to make a fire quickly becomes, the less able you are to do it with improvised methods. If you're injured or wet, inadequately clothed and hypothermic already, everything around you is wet, or the temperature is well below freezing, you absolutely have to get a good fire going ASAP, and you might not be in a condition to go hunting for sticks to use, making tinder from improvised materials, and the time/ability/dexterity to use them.

So my ideal multitool would have, in addition to the usual knife blade(s), saw, and pliers (and a litany of useless stuff such as corkscrews and bottle openers), a firestarting implement. A small flint would do.

The next addition I'd make would be a small LED light, and after that a good quality flat whistle. These wouldn't be my first choices in an unrestricted case, but these wouldn't be too difficult to incorporate into the classical SAK/multitool paradigm (and thus meeting the "1 item" criterion). A P-38 style can opening attachment would also be useful,

After that, my most important difficult-to-improvise concerns (waterproof shelter, FAK and water purification and storage) couldn't be implemented into a multitool except in the most gratuitous way (e.g. carry the tool inside a condom filled with aspirins, bandages and water purification tablets), and even I agree that would count as cheating <img src="images/graemlins/grin.gif" alt="" />

Food doesn't become much of a concern for the first few days, and with a good multitool, decent shelter and a fire there should be no problems improvising solutions to that problem.

Navigation can be done quite well by the sun and stars.

Nature provides lots of materials for building relatively waterproof shelters (though a tarp would probably be the next item I'd add after the whistle once resigning myself to abandoning the 1 item criterion - it can be used for building shelter and water storage, among other things.).