So i always wondered why death knights dont have any aoe spells that require RP, or why they didnt have big hitting spells that chew up all thier rp, so i came up with this (which combines the both).
Carrion swarm
Requires 70-100 (130 with runic mastery) Rune Power. (using all 6 runes grants 60 rp, so you can't use it after using all 6 runes [Unless your using scent of blood]).
Deals X to Y damage Shadow damage (Modified by attack power and rune power used) in a cone X feet infront of them (think dragons breath, but heavier damage).
Now all 3 specs (being blood, frost and unholy) 45 point talents have a 20/40/60/80/100% chance to add a certain "flavour" to carrion swarm when cast.
Now i've only thought of one for unholy and frost (The following ideas may not be to balanced);
Your carrion swarm has a 20/40/60/80/100 chance to grant you a "shield" of locus (i know locus is more a nerubian thing, but bear with me) that absorbs X% of damage dealt, when broken (either via damage, or when the debuff expires [Not when clicked off]) deals Y (based on damage absorbed) damage to everyone in Z 15 yards.
Your carrion swarm has a 20/40/60/80/100 chance for locus to surround you leeching X damage (based on carrion swarms damage) from surrounding enemies for 10 seconds. (think old unholy blight)
Now setting the spec based things aside, i think a hard hitting Rp dump would help (not completely remove) dk tanks problems with dks snap aoe agro (and their aoe agro (frost aside) in general). It will also force death knights to decide between spread out the use of rp (via death coil/FS) or saving it for a carrion swarm.
Share your thoughts on the idea, sorry for bad English.
Tl;DR: spell that costs 70-130rp, does "heavy" aoe damage based on rp used with additional effects based on deep blood/frost/unholy talents.