When it comes to making a new Bakugan...

[RedJLP]

Are you allowed to make a new evolution for their line? Say I want to make a new addition to the Leonidas evolution line called perhaps Sigma Leonidas, is that legal and if so, will his back story have to do with how he evolved?

[Keto]

Yes, you are allowed to in my opinion.
First, there is no rule saying you cannot, second- someone did a similar thing and there were no objections and third- there were no updates saying you cannot. Have fun creating it in the future.