Why does it stand to reason that Dorn will return? I can see an equivalency of (daemon) primarchs being pulled back into the setting, but we have far better candidates for return on the loyalist side. Guilliman was definitely among the living, albeit in stasis and mistakenly identified as mortally wounded (apparently the constitution of a primarch was beyond reckoning - or perhaps Guilliman was a previously unidentified perpetual). Lion El'Jonson is also definitely among the living, even though nobody (other than the Emperor and the Watchers in the Dark) knows. Corax, Russ, Khan, and Vulkan are the other likely candidates since their fates are unknown. Dorn, meanwhile, is largely thought to be dead. The lore has shifted and changed over the years, from him being definitively dead (in Space Marine by Ian Watson), to the vague speculation that he might be leading the Custodes (in the Games Day '99 UK Programme about the Crimson Fists, though no mention of Dorn was made in the 2nd edition lore about the Custodes), to the definitive death in the Index Astartes article (frequently misinterpreted as only his hands being recovered), to the definitive death with only his hands being recovered (in the current lore). And then we have the speculation that Ferrus Manus might return.
Besides, we don't know for certain that the primarchs will necessarily remain. I have a hard time seeing GW give them so easily and then take them away, mind you, but perhaps these mondo characters are meant to last for this campaign only (rolling Magnus into the mix since that whole campaign appears to have been developed as a feeder). I could just as easily see the primarchs and other super characters remaining, but with some limits on their usage (and I'm not going to bother speculating about what such hypothetical limits should/might be).
And since the returning loyalist primach is Roboute Guilliman, the author of the Codex Astartes and individual that forced the division of the legions into the chapters, I don't see a break down of the chapters back into legions. I can see a coalescing of chapters with regard to loyalties (for those whose primarch(s) return), but I definitely see the chapters remaining A Thing.
What I see is a shaking up of the lore, but it will remain largely intact with regard to core issues. The Eldar are feeling a big change in terms of what happens to some of the craftworlds. We're seeing some worlds of the Imperium, including Adeptus Astartes worlds, fall, but this will signal some shifts in a few small areas.
As with any plot progression, some people are going to be unhappy with the changes [when their favorite faction is drastically changed for the worse - Biel Tan players take notice] and others are going to be very happy [when their favorite faction benefits for the better - Ultramarines (Legion) players take notice]. While some of these changes may feel big, for the most part, they'll be mostly cosmetic. Sure, we'll see some new characters and units, some worlds are going to be changed (for better or worse), etc. On the whole, though, the core principles of the setting will remain largely intact. GW is pushing a few plotlines forward, but I doubt we'll see a really drastic revision to the setting [to the point where it will be largely unrecognizable]. It's going to be WH41K, which is just WH40K+ (and a little -).
While bringing a small number of primarchs back into the active setting is going to happen, I don't think that we're going to see a mass return. And if that means that my favorite primarch, Rogal Dorn, remains D-E-D dead, I'm fine with that. Which doesn't mean that I won't be happy to see him return - he's my favorite, after all. But I think there are far better prospects for returning loyalist primarchs and I'd rather see the most interesting (and plausible) for the storyline rather than my personal favorite.
As for the Templars, who knows? It's too soon to tell. From a purely speculative point, assuming Dorn doesn't return, I don't see much of a change for them lore-wise. We may see old characters die and new ones brought forth (or we may not), but I don't see much beyond that.
Just my opinion.
Edited by Brother Tyler, 04 February 2017 - 12:58 AM.
Oops! I forgot about the Black Templars in my answer.