I agree: let's not take Martin's advice here. Martin writes an entirely different style of fantasy than Tolkien did, with quite different themes.
However, having Gandalf stay dead for longer - if not permanently - is quite possible. Tolkien's earliest notes on the plot (published in The Return of the Shadow in History of Middle-Earth) involve Gandalf dying in Moria but returning when the "King of On" is fighting a major battle with Sauron and about to lose. The "Land of On" eventually developed into Gondor, so this is essentially having Gandalf reappear in the middle of the Battle of the Pelennor Fields. Of course, at this time in the narrative's development, Strider was still Trotter the Hobbit, and I don't think Rohan existed.
Metaphysically, Gandalf was definitely sent by the Valar. Still, nothing says they needed to re-send him once he died. Hints in the appendices say they didn't, in fact - he only returned by direct intervention of Eru Illuvatar! Of course, his return gives room for extreme awesomeness and many thematically important moments, but leaving him disembodied beyond the Sea would (I think) emphasize the coming of the Age of Men even more: after Gandalf's departure, normal Men must carry out the plans and defeat Sauron without visible assistance. Even casting down Saruman could be done by Aragorn, I think, without the loss of too much significance. This trilogy would have a different tone to it, definitely, but its themes would be much the same as OTL's.
Or, if vultan wants the Fellowship to have a new magical mentor, might I suggest Radagast? Perhaps, upon hearing of Saruman's treason, he's filled with remorse and sets aside his simple life to return to his original mission as an Istar. This would also fit with Tolkien's themes, in the same way that he made a hobbit from the middle of nowhere be the one to bring the Ring to Mount Doom. Radagast's introduction would have to be handled carefully to avoid making it seem like Tolkien simply regretted killing off Gandalf, but I think it could be done.