the concepts of (non heal) paladins and death knights are exactly the same: warrior who uses holy/unholy magic to strenghten themselves and inflict damage to their enemies. can't get much more similar than that.
yet the actual in-game classes play very differently. in fact, i'd argue a paladin tank actually shares more similarities, gameplay wise, with a warrior tank than with a death knight tank (or at least it was the case in wotlk and cata, i haven't played mop yet).
so the argument that a similar concept means lack of variety and 'new stuff' is pretty bad, since we've already seen it proven wrong -- in an even more extreme example than the addition of a battle mage would be.
1) i've actually repeatedly stated that the plate wearing, warrior/paladin like "battle mage" is only one of several possibilities and that my prefered concept of a "battle mage" class would rather be one of those two:You want a Battle mage? Which is easier to integrate...an entire new class or a fourth spec added to Paladins?
- a leather/cloth wearing and staff wielding one, similar the one in in the original Sacred game, who's main abilities are melee ranged spells that do not use weapons (although i honestly doubt we will see one such as this in wow, due to current class balance).
- one around the concept of the time warden, which shares more similarities with priests, mages and shamans than it does with paladins or warriors.
2) do note that blizzard has, tried to proberly implement the spell-dps-paladin by having the holy-tree support two roles, similar to the feral-tree of druids. in fact, that "shockadin" was officially a possible, playable route for paladins in tbc (at least for pvp) and there were gear sets specifically tailored to it.
blizzard has since decided to abandon that idea, as they could not find a way to make it work proberly in a way that would not change the healing paladin too much but that would still make the spell-dps paladin on par with other dps classes in terms of game mechanics (like having several frequently used dps spells) and power balance.
that of course doesn't mean there is no way whatsoever that they will make it work at some point in the future, but it goes to show how much of a challange it apparently poses to do it proberly (and with resources that might possibly be better put to work on other/new classes).
the above is written assuming that an addition to one existing tree of an existing class is in some way comparable to a complete new class featuring at the very least two different roles (each of which introducing several new mechanics) in three different trees, a new resource system, a new 'style' concept (the death knight 'style' concept, for example, is an undead and necromantic warrior who uses unholy, blood and frost magic) for character, spell and gear appearance, new class-race-combinations and tons of new lore -- which is, quite frankly, a rather ridiculous assumption.
and again, i'm not saying we will never see spell-dps paladins in a working manner. i can absolutely see that happen and i think it would be a great idea, if they can make it work.
nor am i saying we will without a doubt see a battle mage themed class in a future expansion, plate wearing or otherwise. all i'm saying is that one such class is a possibility and that there are several potentionally great ways to introduce one, in a way that would fit well into our current class selection.