I've been thinking about my class and where I'd like to see it go for a while now. I've seen many threads about Dk's lately, and how people are upset with them. Not everyone seems to have the right idea of how to fix them, and I feel what I've brainstormed up is at least a good start in the right direction.
Not looking to make Dk's overpowered, but while I've hit 2200 and obtained my weapon, arena on this character is flat out upsetting, and its not enjoyable to play. I've re-rolled warrior and got 2200 fairly quickly, starting in the 3rd week of MoP's release, and hit 2200 as a subpar comp (TSG) with only 4 pieces of malevolent including T1 weapon. Having played both classes to 2200 this season, I can say that warrior is just flat out more fun. I don't get upset like I do playing a Dk, and its not nearly as stressful.
As far as experience outside of this season, I've obtained gladiator and hero on my Dk.
What Dk's lack
Sticking to melee here. If you look at most other classes in general, in MoP, most of these other melee classes (and classes overall) have had significant overhauls with the launch of 5.0.
Warrior for instance, is a very different class than it was in Cataclysm. This is true for warlock, and enhance and feral have had rather hefty changes to them as well, making them feel different overall.
Death knight, however is the same as were were in Cataclysm. Our rune system is the same, as far as talents go, we have not gained anything new, exciting or interesting other than out level 90 talents, which don't actually provide damage, and are mediocre at best. Gorefiend's grasp is great in RBG's when properly set up with other classes, but as a stand alone talent, its useless.
What Dk's need to become more interesting, viable, and most of all, fun
To put it short and simple, Death knights lack a few things. Mobility, crowd control, and survivability against physical damage.
In my opinion, there are a few things that should be swapped out, and a few things that should be redesigned and reworked.
Death's Advance - Death's advance was originally implemented back when RLS a dominant comp. Specifically rogues (and other classes as well) sat on Dk's all, game every game, because when slowed, they couldn't do anything. Last expansion, frost was considered a gimp spec when it didn't have the heavy damage it did during season 9, because it was locked down too easily. Unholy had deaths advance, and I believe that this should be a core, baseline part of the death knight class. It may be removed as a talent, and something more interesting can be placed in to compete with Chillblains.
Strangulate - This was a fantastic ability during Wotlk, and in some cases, cataclysm (mainly the Cataclysm glyph), back when there was overall less instant CC in the game, the class had the old version of blood tap to provide that rune that is required to use strangulate.
In its current form, the spell is outdated. It still costs a rune, which was designed under the old, Wotlk rune system, and the old blood tap hanging around. It is still on the GCD, unlike most silences, and most of all, has a rather long cooldown for a silence, a whopping two minutes. Strangulate needs to be dropped down at least by one minute, and have its rune cost removed to be even considered remotely comparable to the other silences in the game. It would be nice to see this off the global as well.
Remorseless Winter - This is a neat ability, but there are some design flaws that were overlooked. The biggest problem with remorseless winter, is that it is a five second stun that takes five seconds to activate, and the activation is dispel-able, shift-able, freedom-able, avatar-able, and everything else that removes/grants immunity to snare effects (not to mention CC'ing the Dk long enough to be eight yards away from him for about a second) No one is asking for another shockwave, because shockwave itself is a bit out of control, however this is probably the worst stun in the game. The best thing to do with RW is to either make it break on damage, and instant cast (much like holy paladin blind) or make it an instant AoE stun, and jump it up to a 2.5 or 3 minute cooldown. I think keeping it at a 2 min cooldown is reasonable, but some people might argue otherwise.
If I'm way over my head with my ideas on how to change remorseless winter, then some other sort of CC needs to be given to this class. Being a glass cannon is not enough to get kills these days with all the instant CC running around in arena.