Alignment.
I'm constructing my homebrew right now, and I'm going to modify alignment heavily, because I'll be having a lot of corrupt clerics, fallen celestials, ascended fiends and the like.
Therefore, I plan to do it this way: An outsider's type determines its alignment. A demon, regardless of behavior and morals, is always demonic (i.e. Chaotic and Evil), and will always be vulnerable to spells that affect Chaotic Evil creatures. (I'm also making Fey an 'alignment' type.)
By default, mortal creatures will be considered 'unaligned' except when they are formally indoctrinated (via a ceremony) into a given religion. This is a requirement for divine spellcasters, holy/unholy warriors, some wizardly cults and the like. But actual behavior won't factor into this alignment. 'Alignment' is lost or changed when committing certain forbidden acts/casting forbidden spells/getting officially censured by the hierarchy.
For instance, a Paladin anointed to Palasar remains "Lawful Good" until stripped of his rank and title by a Prelate of Palasar. (Acts which automatically strip him of his alignment include killing his mount or killing a priest or paladin of Palasar. Otherwise, he's a Paladin till caught) So, more than being truly Lawful and Good, many Paladins would rather focus on remaining in the good graces of the Church. And many good and decent Paladins are 'fallen' because they have crossed the path of people in high places.
I'm working this way because I think it makes religion and church issues more nuanced and realistic. (Take for example the Kingpriest of Istar in Dragonlance. I mean, misguided or not, I really think he was acting in an evil manner, declaring genocide and all. But he kept his spells. He kept his rank. Well, at least until the Krynnish gods dropped the meteor on him...)
The way I'll work is this: Most outsiders (including Gods) are aligned according to plane of origin. Metallic Dragons are always 'Good' and Chromatic Dragons 'Evil'. Clerics/Paladins/Blackguards are aligned by deity.
Here's something I'm considering. All Elves are Fey, Chaotic, and Good, unless declared 'Drow' (outcast) by the Elven Court, in which case, they become Fey, Chaotic, and EVIL. And most adventurers will be 'Drow'. Prolonged friendly contact with 'Drow' makes 'Good Elves' unclean. Bearing children with non-elves makes female Elves 'Drow' as well. (Therefore, all half-elves are technically half-'Drow', but don't have the Fey, Chaotic, or Evil descriptors) Oh. I won't have black subterranean spider elves. At all.
And I won't tell the players things work like this! They'll have to learn the hard way. They'll have to learn to be more wary and careful when it comes to issues of morals and when dealing with others. For example, a 'dark elf' will always detect as "Fey, Chaotic, and Evil" and be affected accordingly by spells. A deluded Paladin will always be "Lawful Good", even when he is leading a genocidal crusade. And a fallen Eladrin, no matter how depraved remains "Chaotic Good".