If you lot knew the hours we put into sonme chants, most that never even see the light of day. Its easy to put new lyrics to an established football chant. Much harder to use a new tune. But its worth it, cos if its good every team in the football league will be singing it and you can say - we made that up on the way back from blah blah. Just like Swindon made up Celery (dont believe Chelsea - they are full of shit).
I've often tried while at home on my own, but never being in the company of more than one other fan (my dad) it became glaringly obvious that I was never going to succeed in bringing a new chant to the fore.
I often start something on the spurr of the moment - but as you say invariably it will be to the tune of another already recognised chant.
I will leave my mark one day though.