Sorry if there is a more recent thread of this, couldnt find one.
Mark of the Fallen Champion(1min CD) - A 20sec debuff put on a target, while active death strike heals for 12% (While attacking said target) of the death knights health. If the target dies while marked the death knight gains the 50% amount of damage dealt by death strike while the mark was active in health.
Talented (Deep in the blood tree) Increased the amount death strike heals by 8%, as well as you can put the mark on an allied player, and if that player dies while marked he/she returns to life at 15% health.
Runic Blast(30sec CD/50RP) - A massive blast of shadowfrost damage dealt over 4 seconds (channeled). Damage depends on amount of runic power you have (max of 130 RP) Deals 5k damage per 10 runic power (65000 with 130 RP) with an extra 10k damage if channeled cast is finnished. Deals 50% more damage to targets under 20% health. (Both Frost and Unholy Mastery will be brought into account)
Runic Explosion(40RP/32 Glyphed) - Used as an alternate to frost strike on aoe mobs. Deals a burst of frost damage in a 6yd radius. (Modified by Attack Power and Mastery).
These are just some of the ideas i have for level 90. Mark of the fallen champion would sort of fix death strike in pvp. Runic power/explosion would be an extra ability to spend runic power on.
If you have any suggestions feel free to post them below.