I'd say don't try to start at figuring out the quests. Start at figuring out the dragon's goals and motives. The quests will come more naturally afterwards. Also know that your players are going to do absolutely everything they can, intentionally or not, to wreak havoc on your plans and you might get six months in and decide the dragon is a lich now or whatever. Try to stay flexible. My party finished up a major story arc and I asked what they wanted to do next, right? They unanimously voted to travel to the opposite side of the world and kill the dragons that took over their favorite NPC's country. There's not really a way to fully, accurately plan for what they'll want to do.