Like the Warlock's eldritch blast, the Dragonfire Adept's breath weapon would improve by +1d6 every even-numbered level.
There's no clear mathematical progression of breath effects, so it would seem that Epic Dragonfire Adepts do not gain any after 20th level. I do not think that Extra Breath Effect is an unreasonable feat, even for pre-Epic play.
Extra Breath Effect
Prerequisites: Breath Weapon 4d6
Benefit: You gain one breath effect with a minimum level no higher than your effective Dragonfire Adept level - 5.
Special: You may select this feat multiple times. Each time, you gain access to a new breath effect.
Epic Eldritch Blast suggests a parallel Epic Breath Weapon. Given that breath weapon is governed by Constitution, and is Supernatural rather than Spell-like, I would replace the Spellcraft requirements in Epic Warlock feats with Concentration.
Epic Breath Weapon [Epic]
Preqrequisites: Breath Weapon 9d6, Concentration 24 ranks
Benefit: Your breath weapon damage increases by 1d6.
Special: You may select this feat multiple times. Its effects stack.
Epic Extra Invocation, like the original Extra Invocation feat, seems to work equally well for either class as written.
Scales doesn't initially follow a mathematical pattern, but after 2nd and 8th levels, it increases by +1 every five levels. An Epic Dragonfire Adept would continue this pattern, gaining +1 to natural armor at 23rd and every five levels thereafter.
Damage Reduction doesn't follow a pattern so would not continue to improve after 16th. I'm inclined to fix this by making the Dragonfire Adept's damage reduction follow a pattern; I'd probably settle on 1/magic per 4 levels, starting at 4th.
The epic feats largely do not apply to Dragonfire Adepts, since their invocation list is different and they don't have blast shapes or blast essences. They do suggest parallels, however. Epic feats for Dragonfire Adepts have fewer prerequisites than Epic Warlock feats, so they provide fewer benefits.
Epic Dragonfire Adept bonus feats: Armor Skin, Damage Reduction, Draconic Adaptation, Draconic Insight, Energy Resistance, Epic Breath Weapon, Epic Endurance, Epic Fortitude, Epic Extra Invocation, Epic Toughness, Epic Will, Extended Lifespan, Irresistible Breath, Legendary Climber, Legendary Leaper, Twofold Breath.
Irresistible Breath [Epic]
Prerequisites: Breath Weapon 10d6, Ability Focus (breath weapon), instill vulnerability invocation
Benefit: The Save DC of your breath weapon increases by 2. This stacks with the increase from Ability Focus.
Twofold Breath [Epic]
Prerequisites: Breath Weapon 9d6, at least three breath effects that deal energy damage and at least three breath effects that inflict conditions other than damage
Benefit: You may apply up to two breath effects to a single breath weapon. If your breath weapon deals two kinds of energy damage, half of the damage comes from each type. If a breath effect states that you can combine it with another breath effect, it does not count toward the limit.
Special: This feat does not apply to the Discorporating Breath of Bahamut or the Fivefold Breath of Tiamat breath effects.
Normal: You may only apply one breath effect per breath weapon.
Draconic Insight [Epic]
Prerequisites: Spellcraft 24 ranks, draconic knowledge, see the unseen, magic insight, voidsense
Benefit: You can see through illusions, magical darkness, shapechangers, and other effects (as true seeing). This is a supernatural ability.
You are automatically aware of all spells or magical effects you see. Simply by looking at a creature, you can determine if it is a divine or arcane caster, whether it has any spell-like abilities, and the highest spell or spell-like
abilities it knows. This is a supernatural ability.
When you use the voidsense invocation, you gain blindsight 60 feet.
When you use the draconic knowledge invocation, the bonus to Knowledge and Spellcraft checks is equal to double your Intelligence bonus or +6, whichever is greater.
You receive a bonus on all Listen, Sense Motive, and Spot checks equal to double your Wisdom modifier or +6, whichever is greater. This is an extraordinary ability.
Normal: The voidsense invocation grants blindsense 30 feet.
The benefits of this feat are adapted from the Epic Insight article "Epic Warlock Feats". It's not Open Content, as far as I am aware, but it is available for free from the Wizards website.
In a similar vein:
Draconic Adaptation [Epic]
Prerequisites: Spellcraft 24 ranks, aquatic adaptation, endure exposure, energy resistance, energy immunity
Benefit: Your energy resistance invocation gives you resistance to acid, cold, electricity, fire, and sonic damage equal to your caster level.
Normal: Energy resistance gives you resistance 10 to one energy type.
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