I gave Crash Man haste because he's pretty quick on the uptake, and it's not a wholly non-black ability. I also wanted him to be able to immediately pitch a bomb onto an enemy, if you happen to have seven mana in your pocket. He started out as a red card with a similar effect, but I decided to go for broke and make it destruction, rather than damage. Crash Bomber's thinking is that if you get through to plant a bomb, it blows up anything nearby. Crash Bomber + deathtouch = bad day for your opponents.
Heat Man took some time to try to capture. If you know the battle, you know he zooms around the chamber as a ball of fire, then stops to toss napalm. The way I built him, he feels perfectly red, and very risk/reward. In the right circumstances, you can have him command the battlefield. He's a 5/3 upon entering the battlefield, and can get big fast. I might've even made him too strong, but so be it. Atomic Fire I'm not going to explain. Either you know the game or you don't.
Wood Man has deathtouch because, though it rarely comes up in the game, he basically kills you if he runs into you. He has trample because, if you know anything about me, you know I love using trample and deathtouch to ruin people's days. Leaf Shield gives you vigilance because you can do damage without moving. Pretty straightforward flavor-wise.
Air Man is another card that I used flavor text to develop. I actually made up the flavor text myself as I was coming up with the card, though, so it's really just a card. I like the dual options of his ETB effect. If your opponent has made their way into massive card advantage, you can force them to discard their hand. Otherwise (more likely), you get to draw some cards, which is always nice. The Air Shooter from the actual game is really just a strong weapon, but I like the idea of putting your opponents to a choice.
Bubble Man was one of the last cards I put together. I went a dozen rounds with a dozen different cards, and nothing ever took. I gave him vigilance because he shoots those bubbles that bounce along without him, so it's kind of like attacking but not "moving to attack." I added the Frost Breath effect because I couldn't think of anything else more useful to give him, and it does create some interesting blocking choices for opponents. Bubble Lead is pretty basic. The downside is pretty limited, but flavorful, and allows for a super cheap casting/equip cost (I think).
Flash Man, unlike Bubble Man, was one of the first cards I created for this set. It also had a few different compositions, but the essence was always the same. The flavor is exactly how I felt when I beat him and got his weapon. You can only stop time, but the boss can stop time and then shoot you. What a jerk. Time Stopper, after being broken in several forms, is finally a card I think that has some potential for fun combos. For example, you can equip it to a creature, tap that creature to tap an opponent's creature, then recommission Time Stopper and get your equip cost refunded. Could be neat.