1) Dragons in 2e and 3e (the latter much moreso) were less "dragons" and more "Huge sorcerers with breath weapons". Specially in 3e, they didn't really *play* as dragons, because doing anything other than cast stuff like stoneskin, cat's grace, mage armor, cone of cold, etc, was akin to wasting a round.
Well, some concepts of dragons
are huge sorcerers with breath weapons. I'm less into that, though -- like I said, spell lists and complex buffs aren't something I especially want to make a return (with possible unique exceptions).
2) 3e was the edition that first did away with that problem, by introducing the Xorvintaal dragon template (MM 5), which traded spellcasting for draconic abilities. 4e carried those over as the core dragons (much like 3e first had dragonborn, and the 4e made them core).
Again, ditching the spell list is a good thing. I don't want to track spells/day and rounds of buffs on most monsters, dragons included.
3) Many those non-combat abilities were available in the 4e Draconomicon, in exchange for some of the core ones (blue dragons could get Mirage, for instance).
You say "non-combat," but the distinction isn't as clear for me. When the blue dragon finally confronts the party (if it gets its way), it should be after the party has been lost, starving, and hallucinating with dehydration in the beating desert sun for days. It's abilities make the party arrive at combat already weakened and suffering. In 4e terms, maybe it would make sense to say that there's a whole skill challenge before the fight even begins capable of weakening and draining surges and reducing HP maximums that is vital to the dragon's identity for me -- as important as its lightning breath. 4e's design mentality didn't consider that a key part of the creature's identity. Which is fair enough, but
I do, so 4e dragons failed to meet my needs in that regard.
The illusions and water-destroying and trickery of the blue dragon (just to pick one) are key to HOW it fights the party in the first place to me.
Other dragons aren't very different. A white dragon's ability to call up blizzards and gales, for instance, helps it fight the party by providing concealment and disabling ranged attacks and causing the party to be numb and half-dead from hypothermia, always precariously balanced on the edge of the glacier, before the dragon even makes an appearance.
These are elements I'd be delighted to have 5e bring to the fore -- dragons as holistic challenges. 4e's efforts, like the combat illusion "Mirage" or the combat move "Glacial Armor" do not do that idea justice.
4) It was a simple matter to add a class template (DMG) to any monster, to give them more spellcasting or spell-like abilities. Slap a Wizard template and pick one at-will, two encounter and one daily power and you're done.
Still narrowly focused on the fight, and not really addressing the issue of their thematic abilities.
5) I agree that some of those noncombat abilities could be mentioned easily enough, saving the DM the trouble of making them up (and letting new DMs know of these traditional abilities): "Adult blue dragons innately know a ritual that allows them to foul all sources of water within 1 mile of their location. This ritual has no gp cost and can be used one per week".
Rituals used by monsters in 4e always seemed to be square pegs in round holes -- if you're going to use different rules for characters and monsters anyway, why do they need rituals? Why can't they just
do it?
But more importantly,
what happens when they do it? The 2e/3e magical abilities were flavorful, but they weren't always the most straightforward bunch of abilities. In 4e, it might be treated as a whole Skill Challenge or two to get through these magical hazards and to the dragon itself. In 5e, it's possible that the lair has something to do with this...
For instance, because it's my poster boy, here's kind of what I'd like to see with the Blue Dragon (or at least a sufficiently "legendary" one)
- The ability to create or destroy water used to control a party's movement and rations in the desert. A party going to slay a blue dragon should risk death by dehydration -- bringing along water will not be enough. If the party finds water, and is dragon-savvy, they should know that they drink only because the dragon allows them to (ie, it's probably a trap). If they're not dragon-savvy, they get trapped.
- The ability to mislead enemies with illusions used to gain surprise and keep a party disoriented. A party going to slay a blue dragon should be at risk of getting lost in the desert forever -- a map will not be enough. The party should not trust what they see and hear because the terrain may be a mirage, and the wild animal sounds they hear might be the dragon just out of sight.
- The ability to obscure the area with dust, sand, and grit from powerful winds. A party going to slay a blue dragon should be at risk of dying in a sandstorm, their lungs filled with the desert. Even if they survive, they should expect to be blinded by lashing sand and dust even when they fight the thing.
- The ability to breathe lightning in a line in a fight.
3e blue dragons support this (Air domain, illusion special abilities). 2e blue dragons support this (special abilities including
create/destroy water, sound imitation/ventriloquism/hallucinatory terrain, and
dust devil/control winds). Can't say that 4e blue dragons support this (I could make it work, but it's above and beyond what the books imply I should be doing with a blue dragon).
Will I be able to say that 5e blue dragons support this? I sure hope so.