The belief that OT Elijah would appear after the first coming of Christ is not a novelty in LDS theology. It is part of other theologies, too. For example, Robert Bellarmine (my favourite Catholic theologian to read) said to deny a future coming of Elijah was proximate to heresy. For LDS and even Catholic belief that the coming of Elijah was not completely fulfilled in the person of John the Baptist, but instead, Malachi and Jesus believed OT Elijah would come just before the Parousia, see:
Refuting
the Tanners on the LDS Interpretation of Malachi 4:5-6
New
Testament and Early Patristic Expectation of a Future Coming of Elijah
Robert
Bellarmine on the Future Coming of Elijah
Joseph
Pohle on the Coming of Elijah Before The Lord's Second Coming
Another RC affirmation of such can be seen in Denis Fahey in his The
Kingship of Christ and The Conversion of the Jewish Nation form 1953:
From the Old
Testament, Father Lémann cites first the text of the prophet Osee to which I
have already referred, and which contains the words “they shall fear the Lord
and his goodness in the last days” (Osee, III, 5). He quotes also the words of
Moses: “After all the things aforesaid shall find thee, in the latter time thou
shalt return to the Lord thy God, and shalt hear his voice” (Deuteronomy,
IV, 30), and refers briefly to the prophecy of Azarias in II. Paralipomenon,
(XV, 3-6), and Isaias (X, 20-23). He dwells at greater length, however, on the
last two verses of the prophet Malachias: “Behold I will send you Elias the
prophet, before the coming of the great and dreadful day of the Lord. And he
shall turn the heart of the fathers to the children, and the heart of the
children to their fathers: lest I come and strike the earth with anathema”
(Malachias, IV, 5, 6). “The Prophet Elias, then,” comments Father Lémann, “shall
return upon the earth to bring back the Jews to the Saviour. Our Lord Himself
has clearly affirmed it (St. Matthew, XVII, II). Elias will turn the hearts of
the fathers and the hearts of the children. The fathers are the patriarchs and
all the pious ancestors of the Jewish people, the sons represent the degenerate
race of the time of Our Lord Jesus Christ and of the succeeding centuries. It
is, however, only some time before the second coming of Our Lord Jesus Christ,
before the dreadful day of the Divine Judgment dawns, that our Saviour will end
the prophet Elias to the Jewish to convert them and to save them from
chastisement.
“These precise
statements of the Old Testament find an echo in the New Testament. St. Paul,
who has devoted a whole chapter of the Epistle to the Romans to the conversion
of the Jews, as we have seen, points out nevertheless that this conversion will
take place only near the end of the world. Until then, they continue ‘to fill up
their sins always: for the wrath of God is come upon them to the end.’ (I
Thessalonians, II, 16). The blindness which has fallen on Israel from the time
of the Apostles will remain upon them until towards the end of the world.” (Histoire
complete, etc., p. 463). (Denis Fahey, The Kingship of Christ and The Conversion
of the Jewish Nation [Hawthorne, Calif.: The Christian Book Club of
America, 1953], 106-7)
