Well, how about this set...

Perennias (Ancient Red Dragon)

Head: Crown of the Vengeful Dominator (Attuned) Chest: Insignia of Claws +2 Forelegs: Bracelet of Chromatic Conversion Right Paw: [Legendary] Ring of Maximum Power (Attuned) Back: Orange Drifting Cloak (Attuned)

Crown of the Vengeful Dominator (Rare, requires Attunement)

This crown of twisted black metal has several shards of Obsidian floating around it, and originally belonged to one of the more powerful Obsidian Dragons before it was lost during battle. Replicas have since been painstakingly crafted by other evil entities.

While wearing this crown, all of your spells' Saving Throw DC and/or Hit Rolls gain a +1 bonus for every level a spell is upcast by. (For example, casting Fireball at 4th level increases its' save DC by 1, and casting it at 9th level increases its' save DC by 6)

Value: 9,500gp

Bracelet of Chromatic Conversion (Uncommon)

An item that came directly from the Dragon Queen's workshop, this gold bracelet is set with a Levin Opal that has been suffused with an unsuccessful Chromatic attempt at copying the technology of Crownovi- but even failures have their uses.

Conversion: As an action, you can sacrifice one 3rd level spellslot to the bracelet, giving it one Charge (max 1). You can expend this charge as a free action to turn one instance of Fire, Lightning, Poison, Acid or Cold damage that you can see into Fire, Lightning, Poison, Acid or Cold.

Queenly Adornment: This item is worth an additional 5,000gp.

Value: 5,350gp

Ring of Maximum Power (Rare, Requires Attunement)

This golden ring has a smooth exterior, but it's set with a band of rough translucent diamonds that are normally dull, and occasionally flash with a juddering surge of energy. While many cultures craft these rings on special occasion, they can initially be traced back to the prominent order of Storm Sorcerer Aarakocra who fought alongside Strata in the war of unification.

The ring serves as a +1 Arcane Spellcasting Focus, with the following abilities.

Latent Power: While you are at 50% health or less, and you cast a spell of 3rd level or lower on your turn, you can treat all the spell's damage/effect dice as having rolled their maximum value. Once you have used this ability, you cannot use it again until you complete a Long Rest.

Value: 7,700gp

(Very Rare version)

The ring functions as a +2 Arcane Spellcasting Focus.

Calamitous Power: While you are at 50% health or less, and you cast a spell of 5th level or lower on your turn, you can treat all the spell's damage/effect dice as having rolled their maximum value. Once you have used this ability, you cannot use it again until you complete a Long Rest.

Value: 35,000gp

(Legendary Version)

The ring functions as a +3 Arcane Spellcasting Focus.

Apocdictic Power: While you are at 50% health or less, and you cast a spell of 7th level or lower on your turn, you can treat all the spell's damage/effect dice as having rolled their maximum value. Once you have used this ability, you cannot use it again until you complete a Long Rest.

Value: 135,000gp

Drifting Cloak (Uncommon to Rare, requires attunement)

This cloak's exterior is plain, but the inside has a curious diamond pattern.

When you Dash while wearing the cloak, you move an additional 30ft and do not provoke Opportunity Attacks.

Orange: The radius or width of any AoE you produce is extended by 5ft.

Base Value: 300gp Orange: +4800gp

Perennias was an important lore figure to one of my campaigns, a red dragon that mostly focused on her sorcery. She was originally a pretty unusual Red Dragon because she tended to be more of a defender than an attacker, focusing on keeping people out of her territory. Then she got more exceptional, because she started using her volcanic powers to enrich the soil in her territory, turning it into a sweltering yet lush jungle so she could have effectively unlimited food to raise her wyrmlings with, rather than a barren wasteland intended to starve out attackers. Anyways, when she was about 600, she was approached by a Gold Dragon who'd lost all hope in humanity and gotten almost-blackout drunk, and the two of them ended up in a relationship that started to move Perennias closer to Neutral than Evil, even approaching Good. However, after she ended up firmly falling for him, he went to go and try to help humanity again, and this time, he ended up being killed for his association to her, which sent Perennias into a dreadful enough rage to destroy the very kingdom that her mate wanted to aid and protect.

She was an optional superboss (as you can tell by her insane loadout and backstory), but as you can see it focuses primarily on spellcasting. The idea is that she can enter the fight by dropping onto the party in an ambush (thanks to all that thick jungle in her territory that she can hide in), either kill or grapple the party's squishy mage on her ambush turn, and then either fly off with the mage and murderize them, or brawl in melee for a turn or two before her health gets low, at which point she can combine the powers of her crown and her ring to drop an absolutely disgusting max-damage AoE spell. Normally offensive spells are terrible and useless for dragons because they have their Breath, so this should very easily catch players off guard. After she drops her AoE spell, she can back off and retreat to her lair to heal and get ready for a second attack, if the party somehow survived.