Goku has his Ultra Instinct that he's working on mastering, and Vegeta has his "Limit Breaker" SSB form, so they both enjoy a degree of "uniqueness"
Since DBS Broly is more of a berserker when he transforms..what better form for him to attain than making Super Saiyan 4 canon, and having him as the only Saiyan to reach it? It fits his personality so well as far as the wild feral look...they'll just have to explain how he grew his tail back lol. Design wise, I don't think it should differ too much from his Full Power state other than having fur and a tail. His hair could revert back to black and his fur would be neon green (or his fur could be black and his hair stays neon green) and maybe give him some fangs and turn his eyes and pupils red and make them glow?
Since DBS Broly is more of a berserker when he transforms..what better form for him to attain than making Super Saiyan 4 canon, and having him as the only Saiyan to reach it? It fits his personality so well as far as the wild feral look...they'll just have to explain how he grew his tail back lol. Design wise, I don't think it should differ too much from his Full Power state other than having fur and a tail. His hair could revert back to black and his fur would be neon green (or his fur could be black and his hair stays neon green) and maybe give him some fangs and turn his eyes and pupils red and make them glow?