I get your points about not liking certain hobgoblin or or variations in Volo's which impact upon world building issues. To me that's a personal world building taste issue. I also get making certain monsters tougher (add hp, ac, resistances etc)
1. How have you modified dragons, can you give an example?
2. Why don't you want them to have extra abilities such as were in 4e that are meant to work to redress the likely imbalance in the action economy (such as bloodied breath, a damage aura or breath that combines with a dispel resistance)?
1. I granted adult dragons resistance to bludgeoning, piercing and slashing damage that is non-magical, and ancient dragons resistance to all damage of those types, whether magical or not.
Adult or older dragons also have magic resistance (advantage on saving throws against spells and magical effects).
Adult or older dragons have all of their potential attacks - tail, wing buffet, two separate claw attacks, and bite.
Dragons have spellcasting or spell-like abilities from earlier editions.
I consider the frightful presence to work a bit differently, with creatures having a very good chance of dropping what's in their hands (like AD&D) if they are affected by it. In addition, I go farther than the normal frightful presence in that if you fail by more than 5, even when you've recovered from the effect because you are out of its presence, you find the creature utterly terrifying and would prefer not to go near it at all, but if you overcome your fear to do so, your attacks and ability checks are made with disadvantage when in its presence for the next 24 hours. If you fail by more than 10 (which is the case with most common folk and low-level characters) you'll pretty much be terrified of it whenever in its presence and for all practical purposes, unable to hunt the creature any more. Gaining a level would make a difference, along with perhaps some spells (I'd adjudicate it along the lines of a curse, or perhaps a geas).
2. I don't particularly have a problem with them having additional abilities, although the basic dragon lore in my campaign has remained more or less the same since the late '80s. What I don't like is creating extra abilities to overcome a shortcoming in the rule system, nor abilities that dramatically change the way the creature would interact with the world around it. In the video, the first thing he suggested was an aura of 25' that automatically (with no saving throw) causes fire damage. A dragon wandering the countryside with that effect would have an enormous impact on the world in ways that I find undesirable. It doesn't fit with past lore from the game, and if all a red dragon needed to do was wander the countryside and set it ablaze (which it would do), then it really changes the nature of the dragon itself.
Of course, a dragon can (and should) still have an enormous potential impact, and a red dragon could certainly choose to use its breath weapons to start fires. I should also mention that fire damage is more dangerous in my campaign, potentially causing things to catch fire, and additional damage on subsequent rounds unless you take the time to put it out. Fire is one of the most terrifying things for a person to face in real life, and dragonfire is similar to magical fire in special properties in my campaign (capable of destroying some artifacts, for example, although not necessarily just in an attack). Other breath weapons are similarly more potent, and this also goes back to AD&D where breath weapon saving throws were separate from other types.
I prefer to fix rules that present problems from modeling than create a new rule to compensate. I allow their multiattack to include a bite or breath weapon, two claws (these can be grapples as well), and a tail sweep, plus legendary actions (which deals with the problem of action economy rather elegantly). Note that the tail sweep is an area attack, not just against a single creature, and has a high chance of pushing creatures and/or knocking them prone. A spell or spell-like ability can replace he attacks, or be one of the legendary actions, in addition to its other attacks for the round.
They will know their lair, and typically an area within at least a 10 mile radius very, very well. Their size and strength makes it possible to cause a small cave-in and their size reduces the potential impact of such an event toward them. A dragon's personality with regards to prey in my world are quite catlike. They'll play with the dragon slayers, coming in to attack, perhaps grabbing one or two when flying by, and dropping them from a significant height, trapping them under their claws (a grapple and it has advantage and you disadvantage due to the size difference), only to let them "escape" for a few moments only to do it again. In fact, an initial attack that includes a breath weapon, grappling one or two creatures and then taking off with them is an amazingly effective way to start a combat.
Essentially use tactics in a way to show their utter dominance against such an enemy, until they grow bored. In which case they will either kill them outright, or lure them into a dangerous situation where other monsters (possibly even minions or allies) can "play" with them instead. Most of them are far more interested in a battle of wits/conversation with somebody courageous enough to remain within their presence than to just outright kill them.
I'm not concerned about making it easy, or even all that possible for the PCs to be able to destroy a creature that has survived centuries and is considered to be about the most terrifying and powerful mortal creatures. Fiends are also significantly more powerful than in the MM in my campaigns for the same reason.
Coupled with the fact that level advancement is much slower, and capped by a max level/max level of spell that can be cast, based on your ability scores, a team of dragon slayers is often somewhere between 7th and 11th level, and won't be after an ancient dragon. Most of the time an adventure involving a dragon is for some other reason than attempting to slay it. Dragons on a rampage (which happens very infrequently) can survive a small army, although they don't typically land in the middle of it either. Spells, fly-by attacks, and breath weapon attacks are the most common approach, and it knows when its breath weapon has recharged, so it stays out of range until then. But attacking the civilized world is a dangerous business. A smallish group of dragon slayers is one thing, an organized army that is prepared and well-equipped is a much bigger nuisance, if not ultimately more dangerous. Even then it's more likely to drive the dragon off than to actually slay it.
So it's primarily beefing up the traditional strengths of such a creature, in terms of how difficult they are to hit and kill, how terrifying they are in combat, and utilizing those capabilities to be as effective as possible. Even without beefing things up too much, grappling and taking off with a creature or two and getting out of range of the rest of the party is an extremely effective tactic. A parting shot with the breath weapon as it does so softens up the rest of the party for future attacks. When possible fly-by attacks using grapples, breath weapon, and tail attacks are also a favorite.
My approach is very traditional, but I've always felt that a party that actually succeeds in slaying a dragon should rightfully be legendary. Most of the time it's just another band that's "never heard from again."