Fire Sigils are the flex slots.
Depending on what you're facing:
Nullblade to kill Hooru Control (I'm pretty sure the deck is favoured vs Hooru without any investment, but if its worth doing, its worth overdoing),
up to 4 of your choice of Sashenka/Eilyn/Vara 6 or Vara 7 to grind out Xenan or Stonescar,
Lighthoofs, Lethrai Marauder and more Delivery to fight off TCP.
Don't play Feln if everybody is playing Aggro. You can try Heavy Hail, but realistically nobody should be playing Aggro because Xenan is way too good vs Aggro and Xenan is the best deck by a lot (allegedly, I have never played a Xenan deck and I never will).
Generically good units: Blackmaw >> Lethrai Opportunist >>>> Torgov
Bad but funny cards: Minotaurs, Protective Crow, Hecaton, Hecaton Recruiter
Bad cards: Champion of Cunning
Other cards: Into the Unknown for recursion, Dangerous Gambit for removal, Lightning Strike for removal, Tripwire Trap for removal/face aegis removal.
I'm not a fan of Permafrost in the market. It junks up Feed the Hecaton. The opponent can just sac the unit that's frosted, neutering your removal.
I also disagree with your characterization of "don't play Feln if everybody is playing aggro". There's plenty of great, low-cost removal besides Heavy Hail to tilt the aggro matchup in your favor. Adding some combination of Tripwire Trap, Coerce, Lightning Strike, Vara's Favor, Dangerous Gambit, and Fissure can help you stabilize the board.
I like Mindbroken Cobbler, but he's hard to get procc'd on tempo if you don't have any one or two-cost minions. Otherwise, he's just 4/3 vanilla, in which case I'd rather have Golden Horn Security in that spot.
I rarely win the matchups where I have to use Formbend, more often than not, it just prolongs the loss. Spending 3 power to use it out of the market, just to transform TCP into another unit I have to remove feels bad, man.
Again, just my opinions and observations, I'm not married to them, but I've played lots of Feln this expedition.