I actually don't like Steward in this deck very much, as he's a killable form of acceleration, not to mention it doesn't fixes colors. Vivid Lands, as well, might get in the way more than help, and they don't get you basics, which Terramorphic and the fetchlands do, and which is kind of important, so I think you already lose a bit of tempo, and adding MORE CIP tapped lands would make you lsoe a great deal more, hampering the deck's aggressiveness greatly. However, that being said, I think you might have some trouble playing War Monks, since you have very few islands and ways to get them, so you might need to work on this a bit, or consider losing the Monks altogether (not the optimal choice, though). You could most likely scale back on 1 Gift of the Gargantual for a total of 3, and I think in this deck, with a few expensive spells, Lash Out might be better than Incinerate.
My point about Naya Charm is: Versatility. It's a decent spot-removal (and easier to cast in this deck than Flame Javelin, whch would be the optimal choice at 3 mana), but the other abilites are amazing. It gets you back a Expanse to finish setting up your colors, it gets back a Blaze to help pack things up, it gets you back a
goo beater to start over after a Wrath effect or similar, AND it breakes stalemates by allowing you to tap down their creatures to swing ftw (or buys you time by tapping them before they declare attackers). Also, it does all of this at INSTANT speed, making it an EVEN MORE versatile card (not to mention, it does BOTH these last things, tapping them beofre they attack you WHILE clearing the path, and even if they drop creatures to block, YOU will get to untap, burn them, and swing anyway). Convincing enough?

PS: You might consider playing Bant Charm as well, since you are packing the colors.