DM gave me the go-ahead to play/playtest a homebrew subclass I've been thinking about a while, if it seems okay. I've got the broad strokes down and working on the details, thought I'd present it for third-parties to provide some feedback before I really lock things down.
Warlock: the Spellfire Wielder
Rather than a conventional patron, by birth or divine intervention your being has become intertwined with the Weave itself, and you are capable of harnessing its raw energy for your own ends. Deities of magic such as Mystra may require you to undertake specific tasks as part of this pact, but you are generally driven to oppose abuses of magic, magical endeavors that may damage or destroy the Weave, and protect those using magic or seeking arcane knowledge for the benefit of all.
Expanded Spell List
1st: Guiding Bolt, Healing Word
2nd: Lesser Restoration, Nystul's Magic Aura
3rd: Aura of Vitality, Fireball
4th: Otiluke's Resilient Sphere, Wall of Fire
5th: Circle of Power, Wall of Force
Spellfire Channeling
You do not gain pact magic slots as a function of warlock level; instead, you have a number of pact magic slots equal to your Constitution modifier (minimum of one). You do not regain pact magic slots on a short or long rest. This does not change the number of cantrips or leveled spells known, the level of your pact magic slots, or your spellcasting ability.
Starting at 1st level, as an action or reaction you can end an active magic effect or counter a spell, as if you had cast Dispel Magic or Counterspell at a spell level equal to your pact magic. When you use this ability to successfully end a magic effect or counter a spell that you did not cast, you regain a pact magic slot. You can use this ability a number of times equal to your proficiency bonus per long rest.
In addition, at 1st level you gain the Eldritch Sight invocation. This invocation cannot be replaced by another upon gaining a warlock level.
Spellfire Warding
Starting at 6th level, as a bonus action you may spend a pact magic slot to heal a target within 60'. The amount of healing equals your warlock level + your Charisma modifier. If the ability would restore more than a target's hit point maximum or the target is already at maximum hit points, they gain the remainder as temporary hit points.
In addition, you gain resistance to saving throws against spells.
Overchannel Spellfire
Starting at 10th level, when you would regain a pact magic slot but are already at your maximum number of slots, you may cast your next leveled spell without expending a spell slot. That spell is treated as if it had the empowered and heightened metamagic options if applicable, and you gain one level of exhaustion upon casting the spell.
In addition, when using your Spellfire Channeling ability to counter or dispel magic above 5th level, you add your proficiency bonus to the ability check.
Crown of Fire
Starting at 14th level, you can use an action to summon a halo of spellfire over your head. For one minute or until you choose to dismiss the crown, you gain the benefit of both Fire Shield (warm) and Globe of Invulnerability at a spell level equal to half your warlock level (rounding up), and you may use both your Spellfire Channeling and Spellfire Warding abilities without expending resources.
In addition, any time you successfully save against a spell you regain a pact magic slot.
Warlock pact balance/playability feedback needed