In the interest of being complete, here's the version of the dragon I threw at the party above. I'm quite happy with how it fights, and while I'd have liked to test it in play a few more times, I can only sucker my players into playtesting a monster so many times.
I have to say, I'm quite curious to see what WotC has for dragons in the next playtest. You'd think there would be one, with it reaching level 10...
Red Dragon - Version 3
40 ft (fly 40 ft)
darkvision 60 ft
22 (+6) Dex
12 (+1) Con
18 (+4) Wis
14 (+2) Cha
Common, Draconic, Elven, Giant
Whenever the dragon takes damage, prevent 10 points of that damage. If an attack would beat the dragon's AC by 5, the attack creates a chink in the dragon's armor, and it loses this resistance.
When the dragon has lost its resistance, it fights more defensively. It may choose to spend its superiority dice to parry attacks, reducing damage by the total rolled.
On its turn, if the dragon is in its hoard, it can choose not to move. Instead it can roll in the treasure and fill the weak spot in, regaining this resistance.
The dragon has 2d10 of Ferocity dice, which refresh at the end of its turn. It may spend these dice as follows:
Spend 1 die to deal 1d10 extra damage, and knock the target 20 feet away from the dragon.
Spend 1 die to use any attack.
Spend 1 die to move the dragon's speed.
These dice may be spent between initiative counts, as well as on the dragon's turn.
If the dragon takes more than 25 damage from a single attack, add a d10 to its combat superiority pool. Dice gained in this way do not refresh, and are lost at the end of the dragon's turn if they haven't been used.
Melee Attack - Claw:
Two attacks: +7 to hit (reach 10 ft, one creature). Hit:
1d8 + 6 damage.
Melee Attack - Bite:
+7 to hit (reach 10 ft, one creature). Hit:
2d6 + 6 damage, and the target is snatched up in the dragon's jaws. The dragon may only have one creature in its jaws at a time. If the dragon bites with a creature already in its jaws, that creature is swallowed on a hit.
Melee Attack - Tail Sweep:
+5 to hit (reach 15 ft, up to three adjacent creatures within range). Hit:
1d6 + 3 damage, and each target is knocked 10 feet further from the dragon.
Ranged Attack - Spit Fire:
+7 to hit (range 50/150). Hit:
1d10 + 4 fire damage.
Area Attack - Breathe Fire:
The dragon breathes fire in a 30 foot cone. Any creature in the area takes 4d10 fire damage. Targets may attempt a Dex save (DC 13) for half damage. The dragon cannot breathe fire again for 1d4 rounds.
Ranged Attack - Fling The Morsel:
+7 to hit (range 50/100 ft, requires the dragon to have a creature in its jaws, targets one additional creature). Hit:
2d10 + 6 damage to both creatures. Miss: 2d10 + 6 damage to the thrown creature. Special: The thrown creature falls prone in a space adjacent to the target.
The dragon may use its bite to swallow a creature. While swallowed, the target takes 3d8 acid damage at the start of every turn. The acid damage may not be parried.
The swallowed creature may attack the dragon's stomach with disadvantage, though the stomach's AC is only 16. Swallowed creatures do not interact with the dragon's Impenetrable Hide.