The Main Deck
On average, a pure Fire King deck will have about 16-18 spots for generic non-engine. For the most part you'll want to fill up that space with handtraps like Ash Blossom & Joyous Spring, consistency cards like One for One, and defensive cards like Called by the Grave.
A build focused on going second could forgo the handtraps and defensive cards for board breakers like Lightning Storm and Dark Ruler No More.
Alternatively, you could dedicate some spots for a second engine that helps you reach Legendary Fire King Ponix, like the the Diabellstar/Sinful Spoils cards, or mix in another archetype, like Kashtira. We consider such decks to not be pure Fire King anymore, and as such we will not cover them here. However, keep them in mind if you think you require more power/consistency than the pure variant can offer.
Finally, you can embrace the jank and try running cards like Agnimal Candle in order to make Synchro Summoning possible, whatever floats your boat.
Let's explore some of these cards and see what they can do for us.