It's been rumored recently that GW will be releasing a plastic Horus Heresy standalone game. Once the limited edition kit has run its course, the individual kits in the box are supposedly going to be sold separately as part of the standard 30K game system. Rumors say that the kits will include Mk IV power armor and Cataphractii Terminator armor.
These rumors have been coming from Hastings, a well known and more or less accurate rumor source, and are being confirmed by other sources. Although my opinion of the rumormongers was shaken in 2014, I have to admit that they were pretty accurate with the Blood Angels release. And not only were Harlequins released as the rumors said (a long shot in and of itself), but they were even released in the predicted month.
Honestly, I have no interest in getting into 30K. It's not because I don't like the setting; in fact, I'm collecting the entire series of novels and really like the direction they're going, although they seem to be taking their time with it. No, I'll be skipping The Horus Heresy because a) I already have a pile of 40K models to work on and b) I refuse to work with resin.
However, if plastic models in Heresy-era armor became available, I would buy them up in a heartbeat. Per the fluff, ancient suits of power armor are highly sought after. The most highly regarded and experienced Astartes of the 41st Millennium can be found wearing anything from a single shoulder pad (e.g., Captain Sicarius' Mk II left pauldron) to an entire suit of pre-Heresy or Heresy-era armor. With that in mind, a plastic 30K kit would find itself shared between my Ultramarines' Sternguard, Command Squads, and Veteran Sergeants.
In light of the popularity of The Horus Heresy, it would make sense for GW to start producing the basic 30K troop models in plastic. Hand-poured resin is only economical for smaller production runs. Once you hit a certain point, it's preferable to use plastic injection molding to increase your production while reducing your labor costs and rejection rate. On top of that, GW may want to draw in modelers such as myself who would love to incorporate some older Marks of armor into their 40K army but don't want to deal with Forge World's resin, high prices, or shipping rates. (I am not paying $36 plus $5.40 shipping for five resin Tactical Marines in Mk IV armor that don't even include weapons.)