I actually think there should be 10 alignments: the told 9 alignments plus 4e's Unaligned.
Why you ask? Because Unaligned really covers different ground then True Neutral does. Neutral, as it existed in editions prior to 4e, always came in basically two flavors: characters who were actively neutral and seeking to avoid being "overly" good, evil, lawful, or chaotic, and those who simply didn't care.
The "didn't care" flavor meshes well with the Unaligned alignment, while True Neutral doesn't really.
Also, I'd favor simplifying some of the names. Good and Evil can become the new names for Neutral Good and Neutral Evil, indicating that characters of those alignments aren't actively trying to balance law and chaos most of the time, they're just fully devoted to serve good or evil.
Similarly, I'd rename Lawful Neutral and Chaotic Neutral to simply Lawful and Chaotic, which I think would solve some of the "Lawful Stupid" and "Chaotic Stupid" problems of prior editions, so that players of those alignments don't feel like they have to constantly go back and forth between being both good and evil and instead just focusing on a devotion to order or chaos.
You really nailed this post.
I like that Neutral is like glass: transparent unless there's nothing else to see.
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I think that there should be no required mechanical interactions with alignment. With respect to the Detect Spells I can think of 3 distinct options to define how they relate to alignment, which might serve in most campaigns.
1) They don't exist. Done and done.
2) Traditional, detecting evil creatures and effects.
3) Compromise, detecting only current evil actions, states, or thoughts.
For games with stark moral contrasts traditional might be just fine. For games with Grey-and-Grey morality, or where no one wants to deal with it, getting rid of them is also perfect. For people who like the themes of alignment but hate the headaches the compromise might work.
In my opinion an evil artifact might still show up all the time in the compromise setting, but you avoid every creature walking around with a neon alignment sign. This really simplifies issues in infiltration or betrayal plots, to say nothing of paladins using Detect Evil to justify playing whack-a-mole with random people on the street. When evil creatures do good things, detect good sees it. And vice versa.
I think it's reasonable that a vicious murderer eying his victim in the crowd could be detected, but the same guy grabbing a bagel at his local coffee shop could not. And if he knows the PCs are after him, sometimes the best thing to do is think about his favorite puppy.