Here are the additions for red. I knew I wanted some actual cards at 4, and I wanted some more 1 drop creatures. I was also looking to squeeze in another MDFC with a land on the back, and I decided that a way to exile a dying creature was worth it. I also added a threaten effect, this one with a little discount if it hits the tokens that this cube makes. Speaking of tokens, I opted for more 1/1 elemental tokens and the other hero. Red also got a phoenix and a prowess creature and a cantrip. Bloodrush gets in the cube, and soulbond via a creature that gives firebreathing. I figured that makes blocking difficult and gives red something to pump any excess mana from ramping into. I purposefully avoided other enchantments and artifacts (include treasure creators) because I like red having all of that in the legendary Hammer of Purphoros. It's also the only legendary outside of the gold cycle, so I like keeping it so unique.