I want to avoid Narsil being present on the Feanorian side of any Kinslaying. There are so many other options that don't taint it or make it less than desirable for Elros (or anyone else from who lived in Havens) to want.
If Maglor is going to pass along any thing of his very own, I would vote for his harp. Since he burns his hands so badly he can't play.
I like Brian's ideas about the Eresseans passing it. One of the Lords of Andunie might even be a member of the Guild of Swordsmiths, who continued the practice of forging swords as the most difficult, demanding level of metalwork known to Numenor, with no intention of ever using the swords.
It would also work as a gift during the time when the Numenoreans had started helping the Elves in Middle-earth against Sauron. The Eresseans hear about the troubles in Middle-earth (from Numenoreans, or from Elves who flee West) and wish there was something they could do to help, but by sailing West (or dying) they have been removed from direct participation in history. Passing on one of the finest swords they have left might even be a symbol of the dominion passing (gradually) from Elves to Men. Already in the mid-Second Age, Numenoreans have far more power to resist Sauron than the remaining Elves.
If the Eresseans pass it on, whose sword should it have been, initially? Mablung? Finrod Felagund?