It's been over 48 hours, so the bump roles apply.
Flak Cannons - heh like Barrage
IIRC, this is actually an ability that just enables a separate attack that's present on the base unit's code. So you code in the attack your adding, which need not have splash but could as a Cleave-counterpart, and have the ground attack command be added by learning the ability so you can use it at any range and not just past the default range.
Freezing Breath - Frostwyrms Freezing Breath is already considered OP I think, so on a Hero unit that can't be sniped by anti-air and has faster attack speed than Frostwyrm it's probably gonna be way stronger
Isn't it just a guaranteed stun with the building disable code attached?
Spell Immunity - Boring but a OP passive, I prefer Spell Shield idea.
Not enough for a passive ultimate on its own, being too passive (it does absolutely nothing to change how you play, unlike Crypt Lord or Keeper of the Grove encouraging sticking beefy units in melee instead of flanking ranged units with them), but combined with something that makes it important, like an ability that would otherwise self-damage or fail to damage certain allies as intended, it works out well.
Disease Cloud - Maybe some experimenting with this, Idk how great it would be for a ultimate but if it lasted really long and was more annoying than it currently is it could potentially work
Better to use is the Factory ability to make it a trail of DoT, then do something on death via simple trigger to do the thing when a unit with the debuff dies.
Burning Oil - Well if you didn't notice by now, I think AOE damage on attack makes for a pretty good offensive passive ultimate
The persistent AoE makes me thing it'd work well with Barrage enabling an attack to spread the AoE far better than the default.
Phoenix Fire - Could be quite strong on a melee hero getting extra ranged attacks which casts randomly and on multiple targets leaving them burning for extra damage, put a invulnerability potion on the hero and go for some peasant harass.
No idea how this works, but combine it with a trigger to add some other effect like a Slow or damage debuff and you've got yourself a lovely pseudo-Cleave Ultimate.
Feedback - I'm not exactly sure how this ability works but if you did 30-40 mana damage + whatever hit points damage you do with feedback that sounds pretty strong, include scaling from Claws of Attack you'd be doing Lv1 mana burns per hit. Would probably be much stronger on a ranged hero than on melee, though I think it has potential either way.
Feedback is flat, though, it doesn't improve with the damage offered by items. Unless you set it as a complicated trigger to replicate the effect based on a ratio of the Hero's damage.
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Looking at these, and commenting on them, makes me think of a passive Ultimate for what is decidedly a caster in role (blasting from a fair distance while being squishy and Intelligence based), yet does almost no actual casting:
Rocket Artillery: The {Hero} has mastered the use of rocket shells, offering more room for alchemical charges in their attacks, increasing damage of their attacks and effectiveness of thair specialist munitions. Furthermore, these shells can be reliably aimed in indirect fire across much longer distances, allowing the {Hero} to attack from extremely long distances with even larger charges. This indirect fire, however, has a significant travel time and may miss the intended target. Additionally, some abnormal ammo may have unforseen effects when used in excessive quantities.
(code-wise, you take Barrage or some other method to enable Attack 2, which is set to act much like the various long range siege units do, set a trigger to add Fire at Ground or whatever it's called to the Hero when the ability is selected, have one of the damage-increasing Auras be present to boost damage (don't forget to account for this when describing the alternate attack, as there's no way to view Attack 2 on a Hero in-game), then an Upgrade to alter their other abilities to alter them in accordance with increasing the charge. An explosive shell turns into a sidegraded Flamestrike as it fails to properly detonate, for example, causing more damage
over time, but losing out on up-front damage and thus enabling targets to survive with emergency healing. Sidegrades aplenty, with the details of how it works changing, but the overall role of the ability staying the same.)