Overview

What I have in mind is an adventure that starts at the coast somewhere and follows the river up into Aron's Bowl. My original thought was that there could be a back door into the mountain reached by crossing the Molten Hills. They are rank and foul, but offer an unexpected route to the peak. It is unguarded in an official sense, but inhabited, making it guarded in a territorial sense.
This is a good location for a clan of slaad. I think the trip up the river is one that encounters mountain creatures, bandits, lairs of kobolds and goblins and similar monsters. There are also good opportunities for remote outposts, towers and retreats of cultists and ranger types with their own secret agendas.
My plan is to build an adventure that can be started at 1st level, and gradually built up to a level that is sufficient to face a full grown dragon. This is preliminary, but I think it should be pretty easy to get a party to fifth level by the time the river turns north. This gives opportunity to face far more daunting foes whether they follow the river through the heart of the mountains, or decide to cut off the big loop and head directly across the exposed peaks. Either way, I think you're looking at one or two major lairs of orcs or something similar, and encounters with giants of whatever kind you prefer. The yellow areas are fully mountainous areas, with the brown areas representing the extreme peaks of the Iron Teeth.
In the plateau around Aron's Bowl, the main contacts are with miners. The upper part of the river leading to it is home to many mines. These are constantly tapping tunnel systems that provoke attack from underground creatures. The activity and extracted wealth is also drawing interest from everything in the area. Travel can be difficult, and include encounters with pretty much any level of monster.
The difficulty is going to be narrowing down which monsters to use, etc. Story-wise, I want to have a powerful magic item discovered early on, in the abandoned outpost at the edge of the mountains. The outpost was set up to guard and research the item, but also to keep tabs on a group of cultists in league with kobolds. The cultists have found the outpost and decided it is time to act. They attacked and ransacked the outpost without finding the item they sought. The PCs find it, but have to defend themselves against a follow up assault. This sets up the dynamic of dragon connected cultists that can follow, shadow or intercept the party at different points along their journey.
I feel like at this point I need to break the adventure up into more manageable parts, and plan them by level. I think this will work nicely. My concern is that with a partly civilized area in the middle, the adventure could flatten out too much. I think three tiers will work nicely, with the first getting the party to fifth level, the second taking them to eighth, and the third to tenth. Then the dragon lair itself as a separate encounter (or set of encounters).