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I think leaving growlith the same is fine. If every Pokemon had lots of evolutionary stages, it would not feel as unique. It would also be hard to balance.
I don't think an alternate form of Growlithe would be a great idea, but I am intrigued by the concept of a Growlithe PARALLEL. Kind of like people are speculating on how Shimama is a parallel of Ponyta.
The only thing I would do to Growlithe is maybe have an separate evolution by level. But even then, I really like Arcanine and Growlithe (two of my personal favorites) and I think overall they are good as they are.