I've been thinking a little bit about why the ending to Tales of the Abyss is so open-ended. Sure, it's Tales tradition to have endings that are, at the very least, bittersweet, but it's rare to have one that leaves such an important question unanswered. Now from here on out there will be unmarked spoilers up until the post-credits scene, so consider this a warning, because spoiler tagging everything would make this terribly difficult to read.
This is not a theory on who the mysterious red-headed ending man is. This is a theory on why we can't ever say who he is for sure. There are logical, thematic, emotional, even ship-related reasons to name him as Asch, Luke, or some combination of the two (Lasch, if you will?) Since Word of God left the ending open every decision is valid. But why? Why leave it open-ended in the first place?
My current working theory, besides the producers not picking one to avoid a Broken Base situation, is that it's a meta statement about one of the major themes of the game. There are two intertwined themes that are relevant here - one discusses the ability and right to make your own path, and the other concerns what, exactly, makes an identity. Here's where the meta statement comes in:
Throughout the game, you have been led by the game. No matter what happens, no matter what you, as a player, decide to do, the game will always play out in the same way - Luke will destroy Akzeriuth, kill ten thousand replicas, and sacrifice himself to free Lorelei, and Asch will be a jerk, call you a RRRREEPPLICAAAA and a dreck, and ultimately pull his own Heroic Sacrifice. In a sense, up until Lorelei is free, you, the player, are also following the Score.
The moment where things start to get fuzzy and open-ended is immediately after Luke sticks the Key of Lorelei in the ground and frees the spirit. That very scene is when the questions emerge. Does Lorelei have a hand in resurrecting either Luke or Asch? Does Asch's hand twitch signify that he is still alive, or that Luke's consciousness has somehow transferred into him and resurrected him? Does Luke Disappear Into Light in the end, or is that just a visual effect?
Now think about it - as soon as Lorelei is free, mankind averts its destruction, and escapes the power of the Score - and so do you. Now you have the power to decide what the future of Tales Of The Abyss is. By the power of your free will you resurrect Luke, Asch, or create Lasch - you decide what is important to you as a player, cementing your own identity and morality in the process. The game allows you to do what the characters have done; escape the convention of a linear storyline, assert your own opinion, and make an independent decision about the future.
Have I mentioned this game is brilliant??