I think that it is best to try to make a song out of that riff while it is still fresh so to say. because what frequently happens to me is that I come up with a great, kick -ass riff and if I don't develope it further from that riff stage it loses its power on me with time.
It is like the songs you' ve heard for thousands of times and you get tired of them -- with a riff it happens much faster because it is much simpler
Also another reason for which you might not like your riff as time passes is that it is natural for you to develop with time. In 1-2 years you will most probably like quite different music form what you like now. Also you will be a much better musician from what you are now. Personally I laugh a lot every time I hear something I did 1-2 years ago, not to speak about things I did 4-5 years ago.
So go ahead and make a song... You don't have time to wonder if the others from the band are goig to rip your stuff, you have to take that risk. And if you did one great riff you'll be able to do other great riffs and people will know who is speaking the truth.