A "Judgement" effect that replaces a smite's damage with another, more pronounced, longer term effect.
Maybe flex them towards a "Saint" and give them more ways to spend Lay on Hands points, such as ending spell effects on players and eventually bringing back the dead with a touch.
Perhaps give them a special Magical Weapon as a blessing/gift/manifestation of their Oath, and that weapon gains another magical effect at 11th level.
Could also have them attract Followers and make them into a kind of team or warlord-esque class, where they can give Orders to their two Squires and at 11th level the Squires become Paladins and you have a squad one person RPs as.
You could have them gain Blessings as a result of their Oaths -- something like invocations, or other passive effects that essentially make the Paladin more "holy" at certain levels.
Finally, you could have them become more like Spellblades, and maybe they just flat out can cast spells combined with their Smites, dropping two spells slots a turn to do this but making for some wild turns. Not the wildest idea I've listed so far though.