I'd like to point out that in 700+ posts there hasn't been one halachik source from a Rishon or Achron saying moshiach must be from the living.
Also some more food for thought.
(Sanhedrin 98b):
"Rav said 'If he [Moshiach] is from the living, [then he is] like Rabbeinu Hakadosh [Rabbi Yehuda Hanassi]; if he is from the dead, [then he is] like Daniel, the delightful one.' " (also See Maharsha).
Based on the Talmud quoted above, the Sdei Chemed--an encyclopedic work by the renowned Halachic authority Rabbi Chaim Chizkiya Medini--quotes approvingly (Pe'as Hasadeh, Maareches Ha'alef, 70) a long letter by Rabbi Aryeh Leib Lipkin (grandson of "Hagahos Ben Aryeh" published in Vilna ShaS), where he explains--among other fascinating points concerning the ultimate Redemption--that if we have sufficient merit, then Moshiach will be "from the dead"!
The Midrash (Bamidbar Rabba 11:3) says that the future Redeemer will be revealed, then concealed, then revealed again. This is quoted by Rabbeinu Bachayei and by the Chasam Sofer on the Torah (both at end of Parshas Shmos). The latter writes: "This is a great test that the Redeemer is concealed [Moshe] . . . and so it will be at the time of our righteous Moshiach [that] he will be concealed after [his] revelation, as mentioned in the Midrash."
From the Zohar (Shmos 8b)--as explained in Zohar Harakiya and Shaar Hagilgulim (ch. 13, both by the ARI-Zal, Rabbi Yitzchak Luria, one of the greatest Kabbalists)--it is clear that the man designated to be Moshiach is born naturally in this world, then the soul of Moshiach in the heavenly "Garden of Eden" is bestowed upon him so that he realizes that he is Moshiach, then he becomes concealed, ascending to heaven, and only afterwards is he revealed to the full extent, the whole Jewish people recognizing him as Moshiach.
The great Torah commentator and philosopher, Don Yitzchak Abarbanel (1437-1508), who wrote three lengthy works about the Scriptural prophecies and our Sages' sayings concerning Moshiach and the Geula, writes in Yeshuos Meshicho (Jerusalem, 5753, p.104) that it is possible that Moshiach will be taken from this world and brought into the heavenly "Garden of Eden," continuing: "You should not find it difficult [to understand] that the King Moshiach will be among those who arise in the Resurrection," quoting the above Talmudic passage (Sanhedrin 98b) as proof that this can be so. Here we see the same three stages of revelation, concealment and revelation.