Unholy mastery as a pet project
The biggest problem with unholy's mastery, Dreadblade, besides the passive aspect of it, is the fact that it doesn't directly influence the ghoul pet. Unholy's pet damage is a good sized chunk, thanks to mechanics like Dark Transformation on top of the ghoul being permanent. As a result, mastery ends up being the unholy death knight's least desirable stat in most situations (other than, of course, hit and expertise over cap). So in addition to the possible "boring" factor of the straight unholy buff, mastery fails to service a large part of the unholy ouvre. To fix that, then, an unholy redesign is going to want to make a stab at fixing that.
One possibility is to make a mastery that focuses directly on the pet (much as beastmastery hunters already get). A straight damage buff is possible, but another possibility might be to allow mastery to increase the percentage of Shadow Infusion's stacks by some percentage, with a similar increase to Dark Transformation (to prevent Dark Transformation from becoming a damage loss). This would in theory immediately increase the value of mastery and tie it to one of the most distinctive parts of the unholy rotation, one that is more or less immune from the festerblight nerf roller coaster, as well.
The downside here, of course, is that it would go the opposite direction of the current mastery by making it fail to address the shadow damage on diseases and Scourge Strike. It might also put too much damage in the hands of a pet who can be affected by pathing errors or just die in the wrong situation.
So then, the next option may be to add shadow damage to the pet. A mastery that has a chance to add extra shadow damage to all attacks made by either the death knight or their pet means that it stays with the theme of the unholy death knight and adds extra damage from all sources. The downside is that it's once again a relatively passive mastery. It's just there, and it works. That said, the shadow damage would still feel a little more dynamic, especially if it came with some sort of extra graphic, perhaps a shadow explosion on the mob, or coloring the death knight's swings with a purple shadow aura.
Putting frost mastery on ice
Frozen Heart's straight frost damage buff actually works relatively well for dual wielders. With their Frost Strike buff not only adding more frost damage off the bat but allow them to eschew Obliterate in favor of more Howling Blasts, they do a lot of frost damage. Two-handed frost death knights, on the other hand, get their extra damage from Obliterate, and therefore do less frost damage by design. As a result, even critical strike rating, despite being overwritten by Killing Machine, is a better stat for two-handed frost than mastery. A mastery redesign for frost, then, would have the main goal of finding a way to make it pleasing for both weapon types.
An idea one might mention here is to focus on fixing Killing Machine at the same time. Try this: Make a mastery change such that Killing Machine is no longer an automatic critical strike. Rather, it is tied to mastery, in that Killing Machine will add a set percentage of weapon damage to your next Obliterate or Frost Strike based on your mastery. This allows critical strike rating to be unhooked from Killing Machine and survive on its own merits. You could even make it have a chance to magnify the extra damage from the mastery, making it even more valuable. This could also have the extra side effect of taking a real shot at unseating haste from its unshakable grasp on the death knight DPS throne.
Blood's already a master
Blood Shield is actually arguably one of the better masteries in game. It's dynamic, the player actively participates in making it work, and it provides a noticeable and valuable service in the player's main role. That said, if there's one way to strengthen it, it might be letting help out against those unavoidable unmitigated fatal strikes. At its most essential level, the blood death knight works by healing up from massive damage rather than mitigating or avoiding it like most other tanks do (while all tanks except warriors do have some variety of self-heal, the death knight is the one that makes it the centerpiece of their playstyle). As a result, they are not well equipped to deal with large amounts of fatal, unavoidable damage. Sometimes, they'll die where other tanks would have lived, not solely because of skill, but because they didn't have the ability to mitigate that much damage.
This is where an addition to Blood Shield's mastery might come in, that is, an extra mechanic added to the already solid mastery itself. Let's say, if a blood death knight takes fatal damage, for the next so many seconds, they are kept alive (assuming their healers heal them) and given a chance to make up the health deficit. However, most healing sources won't count toward that health deficit. Instead, they must make up the deficit purely via Death Strike. Whatever Blood Shield amount they would have gained instead goes to clearing that healing deficit. This, of course, would take a lot of tweaking in order to make sure it wasn't either too hard or too easy to make up that deficit, and you'd have to juggle it with Purgatory, so it may end up that mastery isn't the best place to address this specific problem. After all, if it isn't broke, it may be best not to fix it.
Learn the ropes of endgame play with WoW Insider's DK 101 guide. Make yourself invaluable to your raid group with Mind Freeze and other interrupts, gear up with pre-heroic DPS gear or pre-heroic tank gear, and plot your path to tier 11/valor point DPS gear.