This does a few things. First, it sort of ties the new Death Knights together with old in a neat little full circle way. The first Death Knights were created by Gul'dan by imbuing death human knights with the souls of Orc Warlocks. With Arthas creating what might possibly be his last Death Knight in this "old" style, it brings the story of class together in a final way.
Secondly, it opens up some very nice story possibilities assuming this combined being, whom we shall call "Bolfang," is redeemed instead of killed. With Horde and Alliance animosities at an all-time high, Bolfang would be in an unprecedented position to mediate between the two factions. Since his human side is like a brother to King Wrynn and his Orc side is the son of one of Thrall's most trusted advisors, he has an inside line with the two major figures of each faction.
In addition, since Bolvar and Saurfang would in theory share their thoughts and knowledge, their combined life experiences would give them unique insights into each side, and give them a good chance of being able to communicate across cultural barriers and find a way for each side to understand the other. If Saurfang the Younger has even half the wisdom of his father, this is almost a given.
That said, there's a lot of people (myself included, admittedly) who would like to see this Horde-Alliance struggle continue instead of being resolved and forgotten at the end of this expansion. Luckily, there's still a chance for Bolfang to find a new place other than peacemaker: As Highlord of the Ebon Blade.
But wait, you say, we already have Darion Morgraine? I'll be honest here. Darion's sort of a downer. He's so busy being angry and vengeful that there's much else to him. Right now, Death Knights in general have a singular fixation on Arthas and the Scourge. But there's little indication of what happens after Arthas dies. Where do the Death Knights get their purpose? What do they do? I'm not so sure Darion can provide that. He's a somewhat one dimensional guy who just appeared out of a comic book, stole our thunder on the whole Ashbringer questline, and suddenly appears in game as the supreme lord of an entire class, one who spends most of his time brooding.
Now enter Bolfang. Both of these men are not only noble and tenacious, but again, have very strong ties to the Horde or Alliance. Not only is Highlord Bolfang a chance for the Knights of the Ebon Blade to start fresh and shake off the doldrums, but his close ties to the factions can help him manage and shepherd the Horde and Alliance Death Knights have begin to direct them to find new purpose as members of their factions and soldiers against the next great threat to Azeroth.
Bolfang's also sort of nice in that all of this development, no matter what form it takes, can be shown on screen involving two beloved in-game characters. This is less true of Bolvar, since his crowning moment of awesome has been retconned out of the game, but there's still enough of us old-timers around who remember his glory days to give him some love. And regardless, all of this development going on in novels and manga gets old sometimes. The prospect of having a major lore twister like Bolfang get developed and make his debut solely in game would be quite a treat, especially if we get another trailer to go with it.
Kudos again to Callouse for thinking this theory up. I'm really sort of hoping now that Blizzard goes this route.