The question posed prompted me to think of anything from approximately 500 years ago that I knew or had heard, to try and see if looking back gives any indication of what sort of thing endures.
Of what I would term "folk" I only came up with three:
1. "Greensleeves" written and composed by Henry VIII.
2. "Jock O' Braidislee"
3. "Earl of Totnes"
There are obviously loads more, Gregorian Chants I don't count as they were definitely not folk music.
I'd go along with Rap being the most likely western(????) candidate - but I'd also go for Indian, Hispanic or Chinese being the main influence 500 years hence.
One thing I am fairly well convinced about is - that if you've got to plug a whole pile of stuff in, then amplify and mix it to get the required result - it doesn't stand a cat in hells chance - as stated above Joe Public has got to be able to do it, on his ownsome, acoustically if he plays an instrument, without have to rely on one Watt of electrical power.
My opinion only. Only hope there will a similar forum to this because without any question of doubt, folkies in 2502 will still be asking questions, requesting help with lyrics, tunes, etc.