So shamelessly stealing from the Forgotten Realms game setting. I would think that ME would be a great place for a dragon cult. Now toning it down and changing it a bit more for ME, I was considering running a game in the Rhine tribunal and adapting the old adventure Trial by fire. Using the dragon from there perhaps he was discovered by a group of mundanes sometime ago.
These mundanes have been serving and worshipping the dragon as a god. The problem will eventually become apparent because of this worship the dragon seems to be gaining strange powers to influence the nearby town. Subtlely influencing citizens through dreams, etc....
I have some other idea's but the main thing I was considering is because of exposure over time to this dragon and his growing magical powers some of the inner circle cultists may develop strange magic abilities or powers. Nothing like flying or breathing fire I'll have too look through the lists of virtues in the core book.
But I was curious as to what others here might suggest.
I'd consider making him a Faerie, for starters. Benefiting from worship is more of a Faerie thing than a Magic thing, and while both Magic and Faerie creatures can give people supernatural abilities, Faeries tend to be more predisposed towards it.
The dragon war saga seed in Against The Dark could be extended to include a cult, and the two types of dragons (magic v. infernal) ties well.
It would take very little adapting, actually. The really powerful dragons could quite easily stand in for gods; any none of the top tier azdaja would probably be all cool with a bit of dabbling into the role of False God, and if they gaining the kind of power where they thought they had a shot at Fetillusclaviceps, they'd totally go for it and give two claws up to the Pax Draconis.
You even get the lesser dragons serving the 'dragon god' and its whims.
Got to love those infernal creatures who have the ability to mask their infernal nature...
The Soqotrans count here, but I don't think the other two were necessarily cults. There isn't enough info to judge the Atlanteans, and, especially for a magical researcher like Bonisagus, there are hundreds of reasons you might be found obeying an order from a dragon that don't include religious reverence of it. (Also, semantics, but I don't think you can have a cult with one member.)
I've recently rediscovered the Acclimation rules and it does make sense for a dragon to go looking for an individual or group to support and/or guide as that helps them avoid losing Might or magical qualities. So even sticking to the magic route you could build a cult around people benefiting from the dragon's actions.