Personally, the one thing I've been wanting to see since i started DDO a bit over 2 years ago is the addition of the Evil alignment. There's no reason whatsoever that it can't be implemented.
I've heard several arguments against the idea, but here's the debunk for them all
1) This is not a family game. A family game would mean that it's appropriate for all ages, and I certainly ain't letting my 5 yr old play, so that argument is void. This game contains more violence than would EVER be healthy for anyone that has even the most remote problem with telling the real-world from a fantasy world. We kill other humans, we kill other sentient races, and we kill kill kill kill KILL!!!! I think there's one dungeon in the whole game that doesn't have enemies to kill, and your entire objective in there is to completely destroy someone's livelihood. Family game? I think not.
2) Evil wouldn't have an issue with any of the quests in the game. The reason? "Evil sows the seeds of it's own destruction." An evil person would help the Silver Flame out because the Silver Flame is giving them rewards. The Silver Flame is having them destroy the evil character's potential rival for power. And they are the only actually 'Good' aligned faction in the entire game. Everyone else is merely looking out for number one, and giving you a pat on the back for helping with that.
3) Even if you want to say that the Silver Flame, the only actual 'Good' aligned faction in the game, wouldn't hire evil mercs, it's easy. It's already in the game. just implement a small script to change an evil character's Silver Flame favor over to Emerald Claw. And since they are actively evil, according to the Silver Flame, and the two work together, that alone would debunk the theory that the Silver Flame wouldn't use someone it doesn't like to it's own ends. They'd just hope you die while completing the task.
4) Evil is far more likely do degrade themselves to helping people they don't like. Why? Because, you can always call in a marker later, you can always use a little extra cash, and you can always beat someone's face in if you don't like their attitude afterwards.
5) There are plenty of actions in the game that a supposedly 'Good' character can do that could easily be taken as evil, so the mechanics are, again, already there. Example include: Killing off and entire outpost of known-good creatures (you kill off an entire outpost of Eladrin, good aligned part-celestials) in order to find the whereabouts of one person, who turns out to be an evil Tiefling in the end. Not exactly something a good person would do. Killing a man who was trying to save his daughter, and simply made a horrible mistake in his grief-driven efforts (Mentos the Freshmaker... oh, wait... Mentau the Fleshmaker, sorry). Again, not exactly something a good person would do... they'd at least vie for him to surrender and live, rather than taking the course of murder first.
If anyone has more arguments against, feel free to post and I'll do what I can do politely debunk them. Thanks!