In Mishnah Eduyot 8.7, we read the following tradition concerning the then-future return of Elijah and how he will solve outstanding halakhic disputes:
Rabbi Joshua said: I have
received a tradition from Rabban Yohanan ben Zakkai, who heard it from his
teacher, and his teacher [heard it] from his teacher, as a halakhah [given] to
Moses from Sinai, that Elijah will not come to pronounce unclean or to pronounce
clean, to put away or to bring near, but to put away those brought near by
force and to bring near those put away by force. The family of Beth Tzriphah
was on the other side of the Jordan and Ben Zion put it away by force; and yet
another family was there, and Ben Zion brought it near by force. It is such as
these that Elijah will come to pronounce unclean or to pronounce clean, to put
away or to bring near. Rabbi Judah says: to bring near, but not to put away.
Rabbi Shimon says: to conciliate disputes. And the Sages say: neither to put
away nor to bring near, but to make peace in the world, for it is said, “Behold
I send to you Elijah the prophet”, etc., “and he shall turn the heart of the
fathers to the children and the heart of the children to their fathers”
(Malachi 3:23-2. (source)