Was the Two Year Timeskip Necessary? I say no.
Was anything improved because of it? If the Strawhats came back together after the events of Marineford the story would have still worked, if not been better. The Timeskip really killed to momentum of One Piece.
Especially because between Amazon Lily, Impel Downs, and Marineford, the basics of Haki where not only established, quite well intact, but Luffy had unlocked it. So the training was largely unneeded since most of the Strawhats haven't shown that simply adventuring and getting stronger naturally through their journey would have given them enough strength to succeed in the New World.
It would also been better at making Luffy and the Strawhats more of the Underdogs in the New World.
Was anything improved because of it? If the Strawhats came back together after the events of Marineford the story would have still worked, if not been better. The Timeskip really killed to momentum of One Piece.
Especially because between Amazon Lily, Impel Downs, and Marineford, the basics of Haki where not only established, quite well intact, but Luffy had unlocked it. So the training was largely unneeded since most of the Strawhats haven't shown that simply adventuring and getting stronger naturally through their journey would have given them enough strength to succeed in the New World.
It would also been better at making Luffy and the Strawhats more of the Underdogs in the New World.