I just picked up the game yesterday and I’m excited to play it, but I think I’d like to make my own heroes too. Are there any guidelines I can follow to keep the gameplay balanced?
It’s up to you, really. The designers really didn’t focus too much on balancing (except when they noticed something broken during play testing). More they wanted to focus on making the cards play the way they wanted the character to act. E.g. Tachyon is very fast, so her deck is designed to play multiple cards at once; Absolute Zero has a fire/ice thing going on, so cards are designed to allow reacting to fire and cold damage; Legacy is a leader looking out for his team, so he’s designed to help other heroes by increasing the damage they deal or decreasing the damage they take, etc. Basically, create a hero, give him a style, create cards for him that follow that style, test them to see if they are too broken, and, finally, DO NOT POST THEM HERE!! You can if you want, but it will likely end in your posts being deleted (I know first hand… poor Phoenix).
Thanks for the reply!
I’ll make sure to keep my content in my game only! 
Yup, unfortunately the lawyers don’t like posting of ideas here because then it’s hard to prove where actual published character ideas truly originated if there is any overlap. 
However, best of luck on your homebrewed heroes, ghd.