The 4piece t10 which is a flat standard 3% damage increase is just plain dull and boring. Other classes get interesting bonuses although RNG dependant they make rotations interesting and fresh.
What I suggest is something similar for deathknights such as:
Your blood plague has a chance to allow you to use your Rune Strike ability without cost and causing no threat.
Why? Runestrike is a powerful attack, one of the strongest we have and it scales well especially since we're stacking armor pen. It will give you an extra button to press and keep you focused from doing the same old dps cycles. It works well with all trees (maybe notsomuch frost).
Just something I'd like to see, perhaps even as a talent of somesort in cata.