I think the Paladin-Warden-Blackguard is worth a try, but I'm not sure definition by alignment is as exciting as definition by Oath. Not only would it free up alignment restrictions (is alignment still optional?), it also opens up the possibility of creating an interesting union of Power Sources, Patrons, Gods, and Ideals from a number of editions influencing this sort of Paragon or Champion. As someone mentioned elsewhere, you could create interesting design space with a champion of anything from Heaven, Hell, demons, primal spirits, a certain kingdom or civilization, Bushido, Truth, Greed, The Prince of Frost, or Cthulhu. This expanded concept could join together such not only Blackguard, Warden, and Paladin, but also Hexblade (martial-oriented Warlocks). And if the Oath was magical, could we possible see the Swordmage?
Then again, this is a big step in the specificity direction, and if we're going this deep into differentiating martial types, then I imagine we're going to be seeing all kinds of full classes in the future derived from any number of fictional concepts so long as they can justify specific mechanics. It's my personal opinion, then, if this is the chosen design paradigm that the Warlord and some type of Avenger/Assassin hybrid get spots.
I do still want to see a version of the playtest with core four and expansive options to create all other classes within the Fighter, Wizard, Cleric, and Rogue, though, too. Otherwise I'm not sure which I like better.