FormerlyHemlock
Hero
I'm not as skilled a DM as some here, but I'm also not a fan of layering PC levels on top of Monsters, for the most part. I remember certain previous edition's Dragons became, basically, spell casters in scales instead of robes to be challenging. That being said, Dragon's are innately magical creatures, and giving them appropriate abilities and powers, cribbed from spells and feats in the PHB, is certainly appropriate IMHO.
Because of the way spellcasting and concentration work in 5E, dragons will always be more than spellcasters. The base dragon chassis is really strong and that acts as a force multiplier. E.g. the dragon blindsight is key to efficient utilization of 2nd level Darkness or 1st level Fog Cloud. A regular Dragon Sorcerer who quickens Hold Person V has nothing really great to do with his main action after that, only Fire Bolt or Hide or maybe Dodge, but a dragon can quicken a Hold Person V and then target paralyzed PCs with six (!) high-damage auto-crits over the course of the next round.
For instance, that Ancient Red might have an innate Fire Shield (warm version) like ability due to the massive heat and flames it radiates. The dragon's gaze is dangerous, as it acts like Hypnotic Pattern to those who meet it. Those who fall under the effect of the Dragon Gaze are susceptible to the Dragon Sickness (Suggestion, Confusion, Crown of Madness: take your pick). He can use his wings and heat to manipulate the air currents (Wind Wall, with a larger area). "My Wings: A Hurricane!" At his command, the ground around the lair explodes into rock and magma (Erupting Earth). And of course, when he becomes really enraged (half hit points?), he flies into a frenzy of death (Haste).
That is just spit balling off the top of my head, but is more aesthetically pleasing to me. I probably wouldn't want to use all of those abilities, or at least tweak and refine them some, but that is the kernel of the idea.
De gustibus.