I really think Harry… is… Merlin… we will see.
I think that'd have to be an off-stage event. Harry is a long,
long way from doing Merlin-caliber work. Centuries at least; maybe even millennia (n.b. wizards don't
have millennia-long lifespans). Also, the Harry we have seen is frankly not smart enough; he's not stupid or anything, but (for example) Butters in a few months' tutelage from Bob had figured out a bunch of magic that had never occurred to Harry. But Merlin-caliber magic is (that we've seen)
extraordinarily complex.
The only way I could see it happening is if he gets some sort of extra magical knowlege, an Intellectus of magic or etc; and not on a "lab assistant" basis like Bob, as the struggles Bob has educating Harry are well-known. He needs something internal. As I recall, though, Bob wouldn't be enough -- not even Bob understood most of the rune-work, when Harry took him out to look at the island.
Merging with the Archive might do it; or with Demonreach itself (though AFAIK the Archive is bound to a female-descent bloodline, & likely has extremely-potent magic to prevent anyone else from interfering; and a Demonreach-merger would cause a tautological paradox: Merlin creating Demonreach who creates Merlin).
