What would be cool for IAP options, hats. Yes, hats or helmets for Hal, purchased via IAP or in-game coins players can collect as they play. They could have bonuses like slowing down his brain drain, or allow him to bust throw boxes (once per round) or run through the flames (once per round).
I don't like the fact you can buy Golden Brains and keep on running it removes the core challenge in an endless runner game, and the pay-2-win aspect will turn off a lot of users.
I think it's a fun casual game on mobiles.
As for a design critique/feedback:
Think how players would hold their phone, on the left hand, the right hand free to swipe or touch. But swiping means they have to place their finger on the screen and do a motion, when you do that, the finger&hand covers up the right part of the screen so you block out incoming visual cues. The running speed up really quick after awhile and that blocking of whats coming is a detriment to gameplay. Think a fast racing car game, if your hand block what you could see in front of you, you will crash often.
If its a simple 2 small button on the right bottom edge (or anywhere on the right which doesn't block gameplay), ppl can touch the top button to go up and touch the bottom button to go down. By touching icons on the right edge, their hand is away from the screen & the finger doesn't block gameplay. It's also much more responsive than swipe up or swipe down.