You are right. Tchaikovsky did use traditional Russian tunes within his work, ditto Mussorgsky etc and ditto our own Vaughan Williams, Britten, Elgar etc. The former two noted collectors of song.

In fact, the "sampling" of older songs in pop music is in itself carrying on the tradition. When Boney M sang Rasputin many years ago, the drums lifted verbatim from Cozey Powell's Dance with the Devil, it was no different to Vaughan Williams using the tune to A Blacksmith Courted Me for both another trad set of lyrics he collected (Our Captain) and also for his work on Pilgrims Progress (He who would valiant be.)

One wonderful way of writing a folk standard would logically begin with borrowing the tune. Borrowing the lyrics from poems and concocting a chorus with a tune borrowed elsewhere wouldn't be a bad idea. Mike Waterson did that after hearing a poem set to music by Filey Fishermans Choir. I have yet to hear anyone call "Three score and ten" anything but a folk song despite the poem being written and published to raise money for the bereaved. (An interesting Mudcat thread on that, if you are interested.)

The first step in writing a folk standard would be to realise that if it catches on in folk clubs and even gets buggered about with slightly, it is a folk song. Others may or may not call it a classic.

After all, hearing The Watersons sing a chorus song many years ago and more recently people sing it still, the only difference between trad and copyrighted is actually knowing whether it is or not. (Bright Phoebus and Three Score and Ten v Dido Bendigo and I'm a Rover.)

Folk is, in the borrowed words of Harold Wilson, a broad church.