I was thinking about the legendary dragon and then had the following idea. Instead of making some monsters legendary and others not, make legendary a template that can be given to any monster. The template would remove weakness and add other special abilities to a monster to represent their legendary status.
In this way, you could take an orc and turn him into Obold many arrows or a red dragon and turn him into Ashadralon. Count Strhad is not just a vampire, he is a legendary vampire that can walk in daylight and knows all that is going on in his dominion. Drizzt is a legendary drow who does not suffer disatvantage when in bright daylight.
In this way, any monster could be turned into somthing legendary (even Meepo!). Or, if not a template, at the very least, the MM could list standard monsters and then list along with them their legendary counterpart. The idea is that if every dragon is legendary, then 'legendary' kind of loses its meaning. But if you have your standard run of the mill dragons taht you might encounter in the wild, well, that is a different creature than a legendary dragon whos name is only whispered by the brave, or foolish.
This avoids pegion holing dragons, and other monsters, into a role of only boss fight. You can use a dragon as a random encounter without it being an earth shaking cataclysmic show down of the ages with magical inter-planer vortex blah blah non-sense. But then, when you do want a campaign to focus on a wicked dragon which concludes with a big boss fight with him, then, you can kick in the legendary template. And again, it would let you create other legendary creatures as well (I am thinking in particular of what a legendary beholder would be like!).
Exactly how it would work I am not sure, but it should basically let you break the rules so to speak when adding it to a monster. It lets you take away weakness and vulnerabilities, lets you add special abilities (maybe not spells but, spell-like abilities). Maybe it would look something like this:
-When adding the legendary template make the following adjustments:
Increase each ability score by 4 points and adjust attack, damage, defense, hps, and saves appropiratly (i.e. add 2 hps per level of the creature, the creature now does an extra two points of damage, etc.)
-Remove any vulnerabilities/weaknesses the creature has
- The creature has advantage on saves, and if the monster already has advantage on saves vs a particular effect then it automatically passes that save (i.e. a legendary dargon with magic resistance would automatically pass all saves vs magic).
-Any recharge abilities now recharge at a faster rate. So if the ability was recharge on 5 or 6, it now recharges on 4, 5 or 6.
-Add a number of monstr traits to match the story behind the legendary monster (for example, maybe the dragon has the trait "displacment" similar to a displacer beast, and so attacks have disatvantage unless under the effect of a true seeing spell. Or give it "flaming body" like the balor, so that creatures that start their turn within 5 feet of the dragon take 6d6 fire damage).
Exactly how much xp this would increase the monster by would need to be worked out, and it would varry on how many traits you give it, but I think this is a far better approch than just giving a dragon the ability to tail slap 4 times a turn. It would let us take standard monsters and truly turn them into something unique and legendary for our campagin!
What do others think?