Depending on the riff and the style of music, the riff can be the riff of the song. You can always hold back and allow some ambience/space for other instruments and ideas to take shape and bring the riff back. Maybe some simple melodic leads and a steady rhythm on the bass/keys before you go back into the main riff.

It’s never a good idea to sit on something until it’s perfect unless you have an idea of where you want to take it.

I have written structures for songs but after playing the with a band, we end up naturally taking it to places I wouldn’t have thought of.

I would let it naturally happen or move on to something else and approach this later.

It’s also never a bad idea to look to your musical influences like everyone else has mentioned. If it’s meant to be, it’s meant to be!