Breaks continuity with the official timeline, as Gandalf doesn't come to middle earth until thousands of years later. But with the hints they dropped (e.g. the names he mutters and the speaking to insects), I don't see who else it could be. And bringing Gandalf in early for this series would be a good change imho. It would balance out the cast of characters and be an interesting plot development.
Per Tolkien's later writing, the blue wizards arrived during the 2nd age to oppose Sauron in the East, but as far as I know they aren't mentioned directly in the material Amazon has the rights to. Having that role filled by Gandalf might be an acceptable compromise...
