According to John Lindow's Norse Mythology: A Guide to the Gods, Heroes, Rituals, and Beliefs, 'Except for his presence at Ægir's banquet in the beginning of Skáldskaparmál and in the list of Baldr kennings (in "Forseti's father"), Forseti is otherwise unknown in the mythology.'
If I'm not mistaken, the point of Óðinn going out of his way to beget a son specifically for the purpose of avenging Balder's death is the same reason none of the other Æsir did the job themselves: they had each taken an oath centuries before, at the time that they joined their race with the Vanir, that none of them could lawfully take the life of another member of the group. Thus a dilemma was created when Höðr killed Balder, an act punishable by death but which death none of them was allowed to effect.
Váli seems to be the only child of a deity in the mythology who is born to a mortal woman, from Miðgarð (though some sources call her father a Jötunn), and later becomes a deity himself (and his mother together with him). Point being that Váli never took the oath, moreover Forseti was the one dude in Ásgarð who was supposed to be at peace with everyone, since he was renowned for settling all disputes. So perhaps it is telling that he doesn't make an appearance at the Ragnarök: maybe he never participates. And it should make sense for him to join his parents and cousins in the new world after the end of the battle, presumably in a world where there will be no disputes to settle. Maybe it would have been too ironic for him, the peace-bringer, to participate in this cataclysmic last battle, or perhaps it was the one conflict whose opposing parties he hadn't the power to bring to reconciliation.
The only other point of interest regarding him, apart from the notion that he was originally a Frisian god named Fosite, is that his maternal grandfather Nep is sometimes said to be a Jötunn, but Nep is mentioned in the þulur among the sons of Óðinn. In Norse mythology, it wouldn't be strange for the daughter of a Giant to marry a god and become a goddess, but it would certainly be the only instance in which any god apart from Loki becomes the father of a Giant, if the Jötunn Nep is in fact Óðinn's son (keeping in mind that Loki was himself already, sort of, a Jötunn). Moreover it would mean that Balder's wife Nanna was his own niece.