Seriously, check out this sonuvabitch and tell me he doesn't look badass:
https://i.imgur.com/7Mu4zzm.png
To me, much as I dig the LORE of the Forsaken, the character-model just doesn't suit a Death Knight. And yet, I also really dig the prospect of a DK who still chooses to fight for Lordaeron and its people, even undead as they are (shame Sylvanas nuked Undercity, since I always enjoyed "the walk" to the throne room, vaguely hearing the cinematic where Arthas returned).
Shame that was never a supported option. I'd personally have retconned Undead DK's to have been part of the first invasion Arthas made to Lordaeron, thus justifying why they looked different (being raised by Arthas immediately, they wouldn't have decayed).
Then again, if bitch-ass Nathanos got to get a snazzy new human model, what's stopping Death Knights -- literal Necromancers -- from employing similar means?
Not holding out hope or anything, but... as someone who enjoys trying to play Race-Class combinations that are supported by Lore, playing an Undead DK with a Human character-model would honestly be my idea scenario. Well... as long as I didn't have to hang around Orgrimmar ALL the time (although I'm pretty bored of Stormwind too at this point).