[MENTION=6747838]Fastjack[/MENTION], if you want to use Thundertree largely
as is then you need to answer two basic questions:
1. What does the dragon want?
2. What does the Cult of the Dragon want?
Of course, you can expand those questions to ask why they are here, where they are from etc... but you're wanting to keep this simple, as far as I can tell, so you can run the encounter as it is written but have it actually make some sort of logical sense.
The 5E lore on green dragons - which is quoted in full in the
blog post you mentioned - has this to say about the goals of a green dragon:
A green dragon seeks dominion over the forest and treasure, like other dragons. It has a broad definition of treasure that includes the minions and pawns it can use to gain more treasure. Control is its driving desire—control over its environment and every living thing therein.
Back to that in a minute. Let's look at the Cult of the Dragon. What do they want? Traditionally, the Cult's basic goal was to convert a dragon into a dracolich in fulfilment of some weird prophecy. It seems that is going to change somewhat in 5E but I don't have that information so I will stick with traditional Cult objective.
To do that, the Cult must demonstrate to a dragon that it is in the dragon's best interests to ally with the Cult. The Cult lavishes the dragon with treasure and generally helps it achieve its objectives. So, back to the green dragon's goals. It seeks dominion over the Neverwinter Wood. Obviously it's too young to claim the whole thing, but perhaps it has targeted a small elf village not far from Thundertree. (This doesn't have to be designed at all; just make it far enough away - and hidden - so that it isn't an issue. You can also replace "elf" with "gnome" or "pixie" or any other sort of fey creature that you think would work better in your game.)
So what you now have is the Cult helping the dragon in its raids on that village. The cultists in Thundertree are part of a larger group some of whom may be en route to or from that village (which also suggest the possibility of reinforcements). This means you have a more logical backstory for the cohabitation of the dragon and the cultists but you also haven't had to do any extra work.
You can also build further on this. Perhaps the dragon has achieved its objectives and now thinks of the Cult as a liability. Maybe it can polymorph into some sort of talking animal and convince the PCs of the evil of this Cult and persuade the PCs to wipe the Cult out.
However, there's something else in the aforementioned green dragon lore that I think is interesting:
A green dragon’s favoured treasures include people bent to its will, famous or significant people it has subverted (such as a renowned bard), emeralds, sculpted wood, musical instruments, and artistic busts and other sculptures of humanoid subjects.
If the Cult of the Dragon is providing Venomfang with treasure,
prima facie that suggests a pretty significant increase in the size of the dragon's hoard... unless, of course, the treasure they have provided is a renowed bard as this snippet suggests. Further, I would spend a few minutes changing the other treasure in the dragon's hoard to reflect what is described here... and make one of the statues a petrified NPC of some sort: players sometimes love that sort of thing.
How does all of that sound? It will add only a few minutes to your preparation but things now make a bit more sense. Even the presence of the druid is more logical: maybe he is the self-appointed defender of the fey village and he is trying to do his bit to end this threat.
Personally, I think the first appearance of a dragon in a new edition is a time to really pull out the stops and show the dragons as something truly special. I would be drawing on the descriptions in the green dragon lore from the blog post and filling Thundertree with strange mists and whatnot, as well as the warning crows and skulking animals. It might take some extra effort but, IME, players tend to really riff off that sort of extra effort and become better and more committed/interested players as a result.
Or you could just have them walk into a ruined village, fight the Cult, and then get blasted by a green dragon appearing out of nowhere.