It would be a great mitzvah for the ex-husband to remarry his ex-wife – if it is feasible (and he isn’t a kohen and she did not remarry in the interim).
This was a CR thread: Mitzvah to Remarry Your Ex-Wife.
The Jerusalem Talmud Ketubot 11:3 learns this from Isaiah 58:7, where the prophet exhorts the Jew not to “hide from your flesh,” i.e. to look out for our relatives. Rabbi Yaakov says in the name of Rabbi Eleazar that this applies even to one’s ex-wife.
While the Talmud does not explicitly say that the way one takes care of his ex-wife is by remarrying her, many later halachic authorities understand the Talmud to be saying that it is a mitzvah to remarry one’s ex-wife.
See Sha’alot U’teshuvot Tashbetz 3:9; Sha’alot U’teshuvot Avnei Sha’ish 1:42; and Sefer Hachinuch Mitzva 580.
But don’t look for such mitzvos of divorcing your wife just to remarry her.