AOC,
Good to have you back buddy. As Cal said I'm keen to see this moving again.
Plasma:
I think the proposed change to the plasma cannons is good. The AT4+ was a bit much, yes it had some merit with a throw back to 40k 2nd ed plasma cannons firing on high power, but in practical terms the book keeping is a bit much when you potentially have it across a number of formations or times when only part of the formation was in range. Call them Plasma weapons rather than plasma cannon if people have issue with different stats for the same weapons.
@Speaker, mate I see your point but the "abstract" nature of epic probably allows the high power of the plasma weapons to be resolved as a better "to hit" value rather than a infantry specific MW style attack. (ie a plasma has the same chance to hit a squad as a frag missile, but the chance to wound is better, so the combined effect is a better AP value. MW would need require 3 traits: a good chance to hit multiple individual infantry, bypass armour and cause instant death in 40k terms) anyhow.. I like AOC's proposal
Ravenwing and land speeders
Bringing back the MM and HB/AC variants is a good idea, the HB only one sucked IMHO and prevented me from picking them. As I said earlier I'd like to see more done on the Ravenwing, such as looking at the aircraft and unique speeders but for now lets work through the basics.
The ravenwing have had access to the typhoon in 40k for some time now (at least 4th ed) and given the ravenwing rely heavily on the speeders to bring firepower to the fight I strongly recommend adding the typhoon. - at worst the Landspeeder Vengeance could even "counts as" a typhoon.
As for the units, are you going to stick with the Ravenwing attack and Ravenwing support formations? I wonder (due to model limitations/availability) if the Ravenwing Attack could be a 6 strong formation of bikes/attack bikes with the option to add a further 0-2 speeders of any type (at appropriate cost). That way you could deploy the bikers without landspeeders if needed (or lack of speeders) but the option still exists to make a harder hitting formations.
Finally, I presume that even with reverting to MM on the speeders, all bike units will still have "scout"?
Anti Aircraft & list options:
Not now, but down the track please consider the Ravenwing Nephilim fighter and/or the Mortis dreadnought as a fluffy anti-air option. I say this because, whilst its reasonable to have the hunter in the list, technically the DA don't have this vehicle in 40k or the stalker. Further with this list focusing on the "hunt for the fallen" one would expect the DA to have their own unique capabilities in the fight; also with the allies removed this list is limited in air and anti-air capabilities.
I know you don't want this to become a kitchen sink list, thats cool, but don't feel bad about giving them access to a good range of DA gear. After all you have not titans or imperial air at the moment. The risk with making it too limited is the list will only play well a certain way and then it becomes stale.
Alternate VC:
Cool concept, and certainly fits with your idea of the list being the DA on the hunt. I guess you want a rule that makes/rewards you for hunting down enemy commanders and leaders specifically, perhaps more so than the largest infantry formation. Normally a blitz would represent breaking through into the enemy's rear and being able to threaten their lines of communication. In this instance the DA potentially have little interest in fighting the larger war, they're there because of the key leadership in the area (those leader/characters etc on the board) and success will be achieved through their elimination/capture.
I'm thinking a VC could be awarded instead of Blitz if you kill all enemy characters/units with "Commander", "Supreme Commander", or "Leader" but this is potentially very varied. Some list like orks and guard will have heaps whilst a marine list may have few if any.. difficult to balance but then so might be "pick the largest infantry formation". This will take a fair bit of testing

Deathwing Testing:
Sounds good, as above we can test these this week hopefully, families and crappy weather permitting.
Cheers