Some really great ideas and some of them not;
1) As you said, unholy blight should also be changed to synergise with other t1 talents, also i think old Blood Tap should be replaced with Plague Leech.
2) Death Zone change is fine.
3) Desecration is underpowered in that tier for unholy since unholy doesn't use Festering Strike much in PVP, make triggering spells NS,FS,SS (i have no idea about blood). Also i'm not sure about frost if it will ever think about taking Desecration since they will already have Chillbains? And biggest problem for this tier in general is Strangulate is EXTREMELY weak compared to Asphyxiate so Asphyxiate will always dominate this tier. What do they have to do is lowering CD of strangulate to 30 sec, remove rune cost, and reduce its duration to 4 sec. 5 sec stun > 4 sec silence
4) I liked your idea of Death Siphon,
5) Hysteria is cool, Remorseless Storm is great, since army of dead is not usable in arena i have no idea about 3rd talent in that tier.
6) YAY! give me back my hungering cold and screw your remorseless winter.
Also they need to lower the runic power cost of Frost Strike and also lower Frost Presences FS cost reduction so that Frost will be able to switch to Blood Presence when needed without sacrificing HUGE dps. Like, instead FS is 35 RP and FrostP reduces FS cost 15, FS is 30 and FrostP reduces its cost by 10.
DKs have so much potential, I don't think we are very underpowered, but we need some changes to make class more attractive and fun to play, without making the class OP, there is so many thing can be improved, but designers are not playing this game, not considering suggestions, and i don't expect any decent change soon.
Our ridiculous talent design shows how much time they have spent on us.