Neonchameleon
Legend
I'm afraid I got half way through that statblock, threw up in my mouth a little, and decided that there wasn't a single chance of me ever using Niv-Mizzet while making it unlikely I'd even look at Ravnica.Now, are 4e dragons more interesting on the page, absolutely. Now look at this later dragon from the Ravnica book:
Niv-Mizzet
Gargantuan dragon , chaotic neutral
Armor Class 22 Natural Armor
Hit Points 370 (19d20 + 171)
Speed 40 ft., climb 30 ft., fly 80 ft.
STR DEX CON INT WIS CHA 29 (+9) 14 (+2) 29 (+9) 30 (+10) 17 (+3) 25 (+7)
Saving Throws CON +17, INT +18, WIS +11
Skills Arcana +18, Insight +11, Perception +11
Damage Resistances Cold, Psychic, Thunder
Damage Immunities Fire, Lightning
Condition Immunities Charmed
Senses Blindsight 60 ft., Darkvision 120 ft., Passive Perception 21
Languages Common, Draconic
Challenge 26 (90,000 XP) Proficiency Bonus +8
Legendary Resistance (3/Day). If Niv-Mizzet fails a saving throw, he can choose to succeed instead.
Locus of the Firemind. Niv-Mizzet can maintain concentration on two different spells at the same time. In addition, he has advantage on saving throws to maintain concentration on spells.
Magic Resistance. Niv-Mizzet has advantage on saving throws against spells and other magical effects.
Master Chemister. When Niv-Mizzet casts a spell that deals damage, he can change the spell’s damage to cold, fire, force, lightning, or thunder.
Spellcasting. Niv-Mizzet is a 20th-level Izzet spellcaster. His spellcasting ability is Intelligence (spell save DC 26, +18 to hit with spell attacks). He has the following wizard spells prepared:
Cantrips (at will): fire bolt, light, prestidigitation, ray of frost, shocking grasp
1st level (4 slots): detect magic, magic missile, shield, thunderwave, unseen servant
2nd level (3 slots): blur, enlarge/reduce, flaming sphere, scorching ray
3rd level (3 slots): counterspell, fireball, hold person, lightning bolt, slow
4th level (3 slots): confusion, dimension door, fabricate
5th level (2 slots): conjure elemental, polymorph, wall of fire, wall of force
6th level (1 slot): chain lightning, disintegrate, true seeing
7th level (1 slot): project image, reverse gravity, teleport
8th level (1 slot): control weather, maze, power word stun
9th level (1 slot): prismatic wall
Actions
Multiattack. Niv-Mizzet makes three attacks: one with his bite and two with his claws.
Bite. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 18 (2d8 + 9) piercing damage plus 14 (4d6)fire damage.
Claw. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 14 (2d4 + 9) slashing damage.
Tail. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 16 (2d6 + 9) bludgeoning damage.
Fire Breath (Recharge 5–6). Niv-Mizzet exhales fire in a 90-foot cone. Each creature in that area must make a DC 25 Dexterity saving throw, taking 91 (26d6) fire damage on a failed save, or half as much damage on a successful one.
Legendary Actions
Niv-Mizzet can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. Niv-Mizzet regains spent legendary actions at the start of his turn.
Cantrip. Niv-Mizzet casts one of his cantrips.
Tail Attack. Niv-Mizzet makes a tail attack.
Wing Attack (Costs 2 Actions). Niv-Mizzet beats his wings. Each creature within 15 feet of him must succeed on a DC 25 Dexterity saving throw or take 14 (2d4 + 9)bludgeoning damage and be knocked prone. Niv-Mizzet can then fly up to half his flying speed.
Dracogenius (Costs 3 Actions). Niv-Mizzet regains a spell slot of 3rd level or lower.
This is better in but still not all they could be IMO.
The reason's simple. "Niv-Mizzet is a 20th-level Izzet spellcaster. ... He has the following wizard spells prepared..." No. Not only is a long spell list homogenising blandness, frequently overshadowing almost everything else they could do but it's a serious nuisance to prepare as GM and gives me far too much to juggle.
If you want a spellcaster give me a few combat spells - and put the combat effect into their statblock. And that would also allow him e.g. a personalized version of counterspell.