Jack, it sounds like you want a game about *how* the characters are Good, not about Good characters.
Alignment is great for games about Good characters. It's kind of a shorthand: it says "hey, my guy's good, let's get to beating on some orcs, cause they're evil." It doesn't really handle the whys and wherefores.
Games about how characters are Good don't really benefit from saying they're good to begin with. For a game to be about how the characters are Good, they have to have the option of not being Good. Of backsliding and all that. In which case we're in a more nuanced kind of world view. Things aren't black and white anymore.
When being Good or Evil is a mechanical thing, it takes it out of the focus of the game. When being Good or Evil is a void, someplace where players have to make their own calls, Good and Evil can be explored and discussed more, it matters more.
For a mechanical, reward oriented alignment (if that fits the game you're thinking of) I'm a fan of treating them a bit like Keys in Shadow of Yesterday. Come up with clear, action-oriented things that make that character/class good, and have the player take XP for doing those things.
For example, in
Dungeon World, a Good Cleric gets XP for putting the dead to rest or bringing someone back from the brink of death. A Neutral Fighter gets XP for defeating a worth opponent. An Evil Wizard gets XP for binding or controlling others.
The important aspects of doing alignment like this: the player's actions trigger XP, with implicit GM approval. The reason for gaining XP is clear, but applies to a number of situations. The reason for gaining XP is also customized to that class and alignment.