If I'm running wire through the crawl space for say a light switch in a wall above, but I plan on moving said wall 8 feet further away in a couple years is it legal/advisable to leave extra wire coiled as shown in the picture to have it ready to go if/when it is needed? If - as I suspect - the answer is no, is there another way of doing so? BTW, the picture is of coax for illustration purposes, but what I'm asking about would be 14/2 or 14/3 wire. It would be live and in use.
There's nothing against having excess length, but you want to avoid tightly bundled in power wiring due to heat dissipation concerns.
So coiling it up is inadvisable. If you plan to move it 8 feet, run it 4 feet in that direction and then back, rather than making a coil - or 4 feet in any direction and then back, if the 8 feet won't be down in the crawlspace. Use a cable attachment that allows you to release the cable without damage (i.e. not stapling - one of the plastic clip products, perhaps.)