Well I'll have to see what the legendary actions of the Ancient variety are before I really opine on the overall design, but I will say that, whatever the actual mechanical strengths, the lore and lair/regional effects are giving me campaign ideas. I've discovered in my most recent campaign how useful the Dream spell can be for exposition, for villain monologuing, and for highlighting non-face characters in a social interaction, and having a powerful being who can use similar communication at-will on anyone sleeping in its domain has all sorts of potential.
Having local portals in and out of the Feywild bound to its life also suggests a whole set of plot hooks and potential difficult character decisions. I'm a little tempted to have a Feywild campaign start with a party overleveled for the encounter easily mopping the floor with a "strange green" Dragon, taking its horde of mithril and platinum, and then discovering that the portal to the Feywild they unknowingly stepped through to get to its lair has closed because they killed it.
Most of what makes them interesting are the Lair actions, without those the Moonstone Dragon would be an extremely dull encounter, it doesn't even get spellcasting. So you'll never want to use the Moonstone Dragon outside of its lair.
Yes, I think if someone is using them primarily as a monster to fight, rather than as an NPC to interact with and/or a being whose regional and lair effects have worldbuilding impact then they've missed most of the potential.
Not that you couldn't have an awesome campaign based around slaying a moonstone dragon, but as a big boss it is a threat more for its ability to manipulate the party and others while being elusive and at a distance than for being particularly strong in a straight-up fight.
I was thinking maybe a Sleep effect, and then it talks to them in their dreams after they succumb. Maybe an initial save against the sleep effect, and then on a subsequent turn it can affect all sleeping creatures with the effect of a suggestion spell (separate saving throw to resist).
Yeah, I was thinking of just porting in the Sleep breath of equivalently sized Brass Dragons. For this adult that would mean a 60 foot cone of DC 18 con save or sleep for 10 minutes, ending otherwise like the Sleep spell. Recharge 5-6, of course. The DC seems to be based on the Adult Brass Dragon's constitution and proficiency bonus, which are the same as this age of Moonstone Dragon, so it works as a direct copy paste even for those concerned about setting "mechanically correct" DCs.
Nope, although the Ancient Moonstone Dragon might get Spellcasting or something, because outside of its Lair, this one boring Dragon,which means it's no good for a mobile villain or hero NPC. All its Goodies are Lair locked.
Maybe the Ancient Moonstone Dragon will be able to move its Lair around.