Not sure why people aren't talking about this but the confirmed "increase in cost and damage of Death Coil" can potentially be a huge buff for Unholy for several reasons.
1. Increased Death Coil cost leads to increased Runic Corruption proc chance. This leads to more FS and CS usage over DC.
2. Increased Death Coil cost also reduces waste of overflowing Runic Power.
3. Damage increase of DC per GCD.
I know that many wanted to completely eliminate DC off of GCD but I feel that would complicate our already complicated cycle. Increasing the cost & dmg of DC is way better.
Below is the original blue post from Blizz.
Patch 7.2.5 Unholy Death Knight Changes
We are increasing the cost and damage of Death Coil in 7.2.5. Even with no other changes, that change is likely a pure buff (even though it looks neutral), since the resource it's draining is one that was being wasted anyway.
It's not yet clear whether we'll open up further changes in 7.2.5, since the core rotation is generally working well. Having 3 main buttons (Festering Wound, Scourge Strike, Death Coil) that all have no cooldowns, but instead juggle 3 resources (Runes, Runic Power, Festering Wounds), is a relatively unusual melee rotation that we think has turned out pretty interesting. The biggest thing it needs is space to breathe on the resources, as has been discussed a lot. We'll see how it looks with the Death Coil change in addition to the earlier Runic Corruption change, as well as the loss of T19 set bonuses, and see if there's still a problem.
If we need to make further changes, the goal would likely be to focus on talents that cause a very large swing in resource generation or GCD usage. Because the rotation needs to not be flooded for max-level players with endgame bonuses, but also not too slow for leveling players, as was pointed out. This is an issue we're starting to pay more attention to across all specs (especially melee specs). As we have more rotation-affecting talents in the game than ever before, and we buffed a lot of of them in 7.1.5, many talents (or legendaries or similar bonuses) are trying to effect large DPS increases solely through giving more of a resource or proc. It's becoming a more common problem that specs which initially had good pacing get flooded in certain setups, and we want to start correcting those where we can. (Blue Tracker / Official Forums)