Allegiances. Allegiances do pretty much what people expect from alignment and do it so much better while allowing for moral ambiguity when that is part of the setting.
Instead of positioning yourself in regard to Good and Evil and Order and Chaos, you chose a number of groups or ideologies that you feel commited to. If you encounter people who realize you share allegiances, they have a more friendly attitude towards you, and if you have opposing allegiances, it makes relations worse.
But the best thing about allegiance is, that it is an amazing tool to help you play a character who is different from your own personalty. Especially when you play a character in a world, that has very different ideas of right and wrong than your own. When you play a good character and ask yourself "what is the right thing to do", you make that descision based on your own values and customs, but you may not want to do that all the time. Supposed you have an evil summoner taken prisoner and you have to decide what to do with him. Your own values may tell you, that you can't murder a prisoner. But then you could take a look at your characters allegiances and remember that he very loyal to an order of demon-hunters who have the policy of destroying demons and those who work for them at all times. This character has a different view on these things than you do, so killing the prisoner would be what fits the character best.
He may also have an assistant who is also a member of the order, but is so against his will and he doesn't really feel commited to the orders teachings and views. He is a member of the organization, but it is not one of his allegiances, as he isn't really commited or loyal to it's ideology and teachings.
If you want to play characters who have faults and do bad things and are not shining paragons of virtue, then allegiances are a great thing. And I think most people would quite enjoy it. Any time I started a new group, the first time players where the ones the most exited about playing a made up character very different from themselves. It's mostly the more experienced players who lose sight of that and play an idealized hero who represents everything they believe. But many genres have not so shining heroes as a core concept and allegiances can help you get sight of this aspect again.
For my campaign setting I am currently working on, allegiance will be an important addition to the rules.