Some songs are just too good to be as short as they are. Here are some songs I seamlessly extended, with a bit of layering thrown in for flavour.
An extended version of Unite Synchronization, from the awesome webcomic Homestuck.
I did a bit of playing around with the transitions, making different parts of the song segue into each other to keep it fresh and interesting, as opposed to simply looping it. I mostly created this because I wanted it to exist, but why should I be the only one who gets to enjoy it?
An extended version of Incertus from Puella Magi Madoka Magica, with some tinkering and layering near the end.
If you haven't watched Puella Magi Madoka Magica, I highly recommend it! It's a dark and splendidly creative twist on the magical girl genre, and it's probably the best anime I've ever seen.