As it said in the 'shoutbox', any material that has some give. The more modern string materials are too unforgiving and might cause weakening and failure of the bow. Mine came with a Dacron B50 string and as B50 is now difficult to obtain I got B500 and the string I made from that shoots well. Needs waxing regularly but that might be as I shoot in all weathers and the bow (and string) gets wet.
It is because the stretchier materials acts as a damper whereas the newer materials don't. Dacron has some elasticity which absorbs most of the energy left after the arrow clears the bow. The remaining energy is transferred to the limbs. With the newer string materials, all the energy is absorbed by the limbs and if there is the slightest of flaws in construction it will probably cause limb failure. Think about a car that has soft shock absorbers going over a potholed road compared to one that has 'sport suspension'.
Hoyt made a new limb some years ago and were experiencing lots of failures. These manufacturers test thoroughly before they release new products but what they missed was someone trying a new mateial aimed at compounds - no give, so faster cast - and the resulting shock merely destroyed the limbs. Hoyt released a statement advising archers to stick with particular matrials for their limbs.. Hope that helps.
Just saying, but try stretching rawhide or sinew strings, both are traditionally used string materials. Stretch is on average less than 1% of length, this is very similar to Fastflight.
Dacron stretches around 3%, makes it better for doing things like cable backing a traditional bow as its comparable to the stretch on linen or hemp.
Really its personal preference, you have to be more rigorous with checking bracing height on a Dacron string. I shot, and still do shoot a mongol horse bow, its been truly used and abused, and has shot out two fastflight strings in the nearly 10 years its been being shot twice weekly. No sign of let off, no sign of damage.