If a tune hasn't been used by someone somewhere it's probably not much cop. Tunes fall into families, and I'm sure you can get variations which might not have cropped up before.

And you can probably work out ways of harmonising a variant on some tune in one one of the families in a way that will be different, or play it at a different speed or whatever.

But if the sequence of notes you use was really totally different from what anybody hae ever played or sung together, the betting is it would be pretty dire. (There's a few of the around too. Orginality is a guest who sneaks in while you are doing something else, and a welcome guest too - but if you go chasing after her, she'll probably lead you a wild goose chase.)