Dragons are iconic enough that I want something a bit different. First, 1 per size category is great except small, plus 1 "colossal" for 5. Maybe drop medium even; smallest dragons are large. Work out a "colossal" trait that does interesting things (it is Gargantuan size, with extra features; "immune to restrained, paralyzed. When grappled by a non-colossal creature, has full movement and moves the grappling creature along with it. Immune to forced movement. Can break most kinds of blocking terrain treating it as difficult terrain; otherwise ignores difficult terrain." or whatever. 5e has 4 tiers; 1-4, 5-10, 11-16 and 17-20. Dragon's should be roughly tuned to be end-tier boses. A deadly-ish encounter for a group of 4 end-tier characters, with a bit of rounding to make the gaps even, works out to roughly: Large: CR 7 (10') (deadly for T1) Huge: CR 14 (15') (deadly for T2) Gargantuan: CR 21 (20-30') (deadly for T3) Colossal: CR 25 (40-80') (deadly for T4) Titanic: CR 29 (100'-200') (because sometimes you want a dragon the size of a hill) Next, modules you can bolt onto dragons. Want it to be a badass example of a dragon at a tier? Bolt on this bit. Rather than a bunch of sub-adult mid-way categories, I'd rather modify the previous category with 1 (or maybe more) "badass" boosts. Those "bolt on" additions should modify CR in calculated ways (per dragon), and be intended for either higher optimization parties, fighting at different spots in the tier, or larger parties. One "bolt on" should be Mythic, as an example; it doesn't adjust CR, just makes you face two encounters of the same CR in a row.
