Currently thorns is a flat amount of damage returned upon being stuck by an enemy attack. It has no proc coefficient and basically only does what it says on the tin. That's a blessing in disguise since it means it will be quite easy and painless to implement a change.
INTENTION: As I see it, the intention of thorns was to provide a source of passive damage. Presumably it would be good for tank builds that deal less active damage.
Addressing the next few points is more complicated. I think this is best done on a character by character basis and not as a systemic one-size-fits-all cross-class solution. Let's see how we can bake thorns into skills.
In the case of thematically similar skills, like Ice Armor: Jagged Ice it would be dangerous to add it in directly and multiply the effect, so perhaps address the passivity. Allow reactivating your Ice Armor to immediately apply your thorns damage in the surrounding area. That turns it into an active ability and gives you a place to tune it without messing with too many moving parts elsewhere. Consider avoiding scaling the damage with normal damage scaling stats. The point here is to open up a new playstyle.
A good active skill that could benefit from thorns is Furious Charge. Thematically it makes sense, so let's go from there.
Rush forward knocking back enemies and inflicting thorns damage as well as 195% weapon damage to enemies along the path of the charge.Now we can actively apply thorns damage as a barbarian as well.
As for scaling into the endgame, that is tricky since the game assumes scaling that isn't purely linear. Obviously, we need a factor of multiplication. I personally lean toward vitality as a scaling stat for thorns. Currently, a high thorns score looks like 10,000 damage and a high primary stat looks like 3000. Most people keep their vitality at around 1k, so a balls-to-the-wall thorns build would deal 300k and a normal vitality score would deal 100k, but only in this extreme case of the stat. Seems about right to me since you still have your primary damage sources.
EXTRA PASSIVE IDEAS.
Just for fun, here are some extra passive that could use thorns and might add some fun to thorns builds.
Wizard - College of War: Gain additional thorns damage equal to the sum of your strength and dexterity stats.
Witchdoctor - Quiver of Nettles: your thorns attack deal an extra 10% weapon damage.
Monk - Cloistered Monk: Increase thorns damage by 30% when equipped with a staff. Increase Mantra of Retribution effect by 15%
Demon Hunter - Siege Tower: Deal double your thorns damage if an attack was blocked.
In any case, let's keep the discussion going. I'm interested to hear your thoughts on this. Try to focus on concepts instead of specific numbers where they aren't needed.