The only Horde Race that would make any sense as a Paladin are Pandarens.
Gnomes and Night Elves Surely? Both can be Warriors, both can be priests - so why not the combo in the form of a Paladin?
Many that live deserve death. Some that die deserve life.
And what religious organization does NOT welcome new initiates with open hands?
Also has the advantage of not messing up with the faction balance since both factions get access to Pandaren. Not to mention they already have access to the "horde class" shaman
If Zabra decided to offer assistance to Darkspear and train up some of them in the ways of the Paladin, at the end of MoP, then by the time WoD comes around, there could be sufficient numbers for Trolladins to be acceptable.
A similar argument could be applied to most races, in lore, for Paladins to be accepted. The argument made by Eddrik could be used to push forward with Undeadadins, or perhaps Sylv offers (re: controls) the last pockets of Risen to join the Forsaken to supplement her forces.
If Blizzard decides to go head-on with the Army of Light thing, there will be a need for the Forsaken to become closer with the Light in some way and there are Undead Paladins out there who could offer assistance in such matters.