... I also think that Harry will find a way to the island that will get him there almost instantly. If not a way, I wouldn't be shocked if he learned to teleport in next book or two. Why else bring it up like Jim did in Battle Ground? Harry can improve his shield bracelet and the other gadgets he already has, but honestly I believe he has all that he needs ...
I think Harry needs more "gadgets" for two reasons:
1) To cover his weaknesses. He's really really strong, in the areas here whe's strong. But he's only fair-to-middling in several other ways. Finesse and delicacy are areas where he still needs these "crutches." Also note that the Merlin -- in "combat gear" -- looks a helluva lot like a "Special Forces" guy, wearing a harness and a BUNCH of pockets, pouches, clips, etc holding a BUNCH of these gadgets.
1a) Harry's chiefest weakness is just his frail humanity. He still dies when someone shoots him, stabs him, etc. His shield-bracelet can stop a
hail of bullets... if he has a moment to deploy it. Someone shooting him from under a veil would be an insta-kill. Likewise a savage brawler like a ghoul: if veiled, if Harry had no clue until the ghoul actually had his claws on Harry... that'd be it. Remember Eb's robes, in the battle at Chichen Itza? Eb didn't have to "do" anything, the bullets just thumped into the robe ... and dropped to the ground. Harry needs defenses that are always-on, that defend him even if the bad guys give him no warning.
2) Fan service. The readers just
like Harry's toys. They like the scenes crafting them in the lab, where Harry and Bob snark at one another.
... We've yet to see fully what the staff he made from wood on the island will do.
The easy answer would be, "open a Gate from the staff to the tree." Boom, on the island.
The other likely method, IMHO, would be to find Runes linking the castle to the island.
I mean... the island's Runes were a level of complexity even
Bob found daunting. And now Harry's
living in a castle carved with such runes... and Bob is living *AS* the castle. Talk about a chekhovgun!!!