This is a list of quick ideas I put together when one of my group decided to play a dragon shaman in my Eberron game. I think it'd be easy to come up with ten similar concepts for any other setting, then let the player do the work of ironing out the details.
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1 – You’re part of a small dragon-worshipping sect, who may hold you up as a priest, hero or divine agent.
2 – You’ve escaped from a small dragon-worshipping sect, taking with you the secrets of their power. They may or may not be hunting you.
3 – You met a dragon in your travels who convinced you to work as his agent in mortal society. The dragon can be a patron in the campaign if you like.
4 – You found a dying dragon in your travels, who bequeathed a measure of his power to you before expiring (or perhaps you stole it from him). Other dragons may not be happy about this.
5 – There are places in the world where parts of the Draconic Prophecy becomes physically visible in the landscape. You saw this happen, or interacted with one of these greater marks, and it changed you.
6 – The Prophecy states you have a special destiny, and your abilities have arisen spontaneously to protect you. Dragons may be very interested in you, in both positive and negative ways.
7 – You uncovered an arcane rite that lent you a measure of draconic power, and possibly tied you into the Prophecy in the process. Others may seek to gain the same knowledge from you.
8 – An ancestor masqueraded as a human to infiltrate the Seren barbarians, who blessed her with draconic power that has passed down to you.
9 – An ancestor masqueraded as a human to infiltrate the Seren barbarians, who cursed her for her crime. That curse manifests as power in you, but there may be a price to pay.
10 – Who cares where your powers come from? The past is the past (and won’t be relevant in play); what matters is what you do in the present and future.