Thanks for the comment, and the suggestions!!
I think it's definitely a judgement call between Gathering and Outburst. Personally, I prefer Outburst because it's always efficient- 3 bodies for 3 mana- and doesn't get weakened if I
flash it back with Chandra; but Gathering is equally strong in the slot and has the potential to give more value by copies 3 and 4, so could be worth a shot!
I'm not actually a huge fan of
Slaying Fire in the StF slot. Convoke is way stronger than it looks, and I'm often Stoking for free after playing a token spell or two first, which is an option that isn't available with
Slaying Fire. Stoke just lets me be more Mana efficient and also tricker around combat.
Steam kin is another toss up for me. It would definitely be powerful, but the fact that it needs three spells to be case after it to give me my value back feels like a lot in this deck. We're not often casting 1 Mana burn spells, so takes longer to build up counters. In contrast, Prospector is cheaper (filling in the 1 drop slot) and synergizes well with the rest of our plan. Could def see Steam kin being good though!
Finally,
Young Pyromancer is honestly the most intriguing card for me. I really like the utility of curving Prospector into
Instigator into a 4 drop, but Pyro might be worth it for the sheer power it provides.
Will definitely take a look at your deck too!