Hello! Ganjo here!
Moclarke, you're right...but only in the context that you can imagine and create anything; it's your mind and thoughts. But you cannot make that decision of anyone else's creation. Y'see, Hiro Morita is creating a manga from someone else's existing idea; there's no restricting his creative freedom under this contract, he can make characters left & right, but Takara-Tomy owns the intellectual property (I.P.) and Hiro Morita is contractually-obligated to work within the terms of whatever agreement was made between them. [I'm not gonna get into the legal stuff; that's too much. (⌒_⌒;)]
Put simply, Mr. Morita was hired by Takara-Tomy (and/or CoroCoro Comics) to create material (art), content and stories centered around the IP. But not the right to the IP itself; which would allow him to change whatever he pleases.
It's a lot like you loaning someone something; there are certain conditions (hopefully in place) to make sure that whatever is being loaned is used properly. Mr. Morita, by no means, owns the IP of Beyblade Burst, nor Toei Animation, for that matter. They're just using the IP of Takara Tomy for business purposes.
That being said, if the contractor (Mr. Morita) presents an idea (referring to his right to create) but the client (Takara Tomy, being the creator of Beyblade Burst) doesn't approve, it's their right as the legitimate owner of the IP to do so.
I know you feel strongly about how things should be, but you can't tell someone else what to do with their own. Maybe one day, they'll create more diverse characters, but that's not anyone's decision. Otherwise, we would be restricting someone else's freedom to create what they want.