The Gollum team, while they didn't have access to Ian McKellen, create a perfect version of the movie's Gandalf.
Lesson For del Toro:
With McKellen to hand, this should be a no-brainer.
Yet we also want to see the film explore other sides of Gandalf: his younger, slightly more impetuous side and how his relationship with Bilbo developed to the point we see in Fellowship.
McKellen's up for the acting challenge of playing a younger Gandalf. But stick to the idea that he's near-ageless and skip the awful plastic youth-o-tron used in X3 and Wolverine.