Obryn
Hero
You won't get everything to work just like it did under 3.5, since these are very different games. With that said, I think you could pretty easily do a four-tiered approach.
(1) Backgrounds. Everyone picks a background which goes along with their alignment. This gives a pretty minor bonus, like +2 to a skill or something of that nature. However, it qualifies you for ...
(2) Feats. Characters can take feats which enhance their alignment's specialty, so certain ones could get feats which enhance healing; others might get better damage on weapon attacks; others might improve elemental attacks of various sorts; others might work as element-swap feats, so you could change (for instance) fire damage into acid damage.
(3) Power-swap feats. Each alignment could have multiple powers. You could take a power-swap feat to trade it out - kind of like Spellscarred feats, only without a Multiclass Feat requirement.
(4) Paragon paths. Give each alignment 1 or 2 paragon paths which work with their stuff.
If this is done well, I wouldn't bother making any restrictions on what alignments can be which classes. The feat chains, powers, and paragon paths will make it pretty clear which ones work best. Additionally, reflavoring the existing powers shouldn't be too rough... Take the Cleric E1 power, Healing Strike. A holy character would smite their foes and heal their ally with the grace of their god. An unholy character would attack their foe, and siphon that foe's life energy to their ally. Same game effect, totally different flavor.
-O
(1) Backgrounds. Everyone picks a background which goes along with their alignment. This gives a pretty minor bonus, like +2 to a skill or something of that nature. However, it qualifies you for ...
(2) Feats. Characters can take feats which enhance their alignment's specialty, so certain ones could get feats which enhance healing; others might get better damage on weapon attacks; others might improve elemental attacks of various sorts; others might work as element-swap feats, so you could change (for instance) fire damage into acid damage.
(3) Power-swap feats. Each alignment could have multiple powers. You could take a power-swap feat to trade it out - kind of like Spellscarred feats, only without a Multiclass Feat requirement.
(4) Paragon paths. Give each alignment 1 or 2 paragon paths which work with their stuff.
If this is done well, I wouldn't bother making any restrictions on what alignments can be which classes. The feat chains, powers, and paragon paths will make it pretty clear which ones work best. Additionally, reflavoring the existing powers shouldn't be too rough... Take the Cleric E1 power, Healing Strike. A holy character would smite their foes and heal their ally with the grace of their god. An unholy character would attack their foe, and siphon that foe's life energy to their ally. Same game effect, totally different flavor.
-O