CapnZapp
Legend
All your ideas about cover, and cool environment effects and such are great.Here's the thing. A dragon is only as weak or as strong as the circumstances under which it is fought allow it to be and how the DM plays it. If played to its full potential, a dragon is far more deadly than many people might realize.
For example, if the dragon is already in flight when the battle begins, then it can swoop down and pick up PCs with a grapple to toss or drop them with great force, or use its breath weapon from a distance.
For example, if the dragon is being attacked in its lair, then there are any number of traps and lair actions that could greatly hinder the PCs from even reaching the monster.
For example, if the environment is destructible or mutable then the PCs have to worry about things like falling debris, or burning trees, or cracks in the ground, or difficult terrain.
For example, if the dragon is attacking in an area with lots of obstacles, then it can potentially use cover against the PCs.
Grappling, flight, environmental obstacles, hazards, the improvised damage table in chapter 8 of the DMG, lair actions, traps, and falling damage - it is easy for a DM to overlook any one of these aspects of the game, thereby making a dragon fight easier than it would be if playing an intelligent dragon as it might logically behave under reasonable conditions.
But the "play it as an intelligent monster" bit is an old hackneyed trope. A red dragon is the king of the hill. It will not retreat. It will not sneak. And it will definitely not use cowardly (and frankly, boring, tactics) such as taking one party member and fleeing.
More generally, if we want a monster to do special stuff, that stuff should be in its stat block.
A designer should not be able to get away with "yeah, this monster is a bit vanilla, let's raise its INT from 10 to 20, that should do it".
That does nothing. Except give it better skills and INT saves. For some posters, however, it inexplicably green-lights putting the onus of making the actual monster work on the DMs shoulders, and that's not what we pay the designers for.