--SNIPPED AWESOME COMPANION SPECULATIONS--
Ok, tried to trim that down so it won't take up too much space. I need to learn how to chop up quotes in this new system, 'cause I kind of want to address your post point-by-point, hahaha.
First, it's TOTALLY freeing to finally get all of this speculation off of my chest, too! Glad to meet a kindred spirit. Now, onto the meat of this post...
So, David Gaider and his two companions, two romances. I've certainly given this conundrum a bit of thought, and I don't really have a definite answer. All I know is he's for sure writing Cassandra, and a "kind of companion that he's always wanted to write." That could, of course, refer to Cass, but perhaps not. I still think Cole has the best chance of being the Asunder character, although I'd love if it were Evangeline I think she and Rhyse are a pair that depend too heavily on Wynne definitely being alive (which she might not be in some players' continuities). Cole is a character that can stand relatively independently of any canon out there as we enter DA:I. However, as you noted, that leaves us with a second romance, since Gaider said Cole definitely would not be romanceable if he were a follower (good move, DG, good move). The only three viable alternatives, in my eyes, are that David's second romance is a Traynor/Cortez style romance, that DG is writing an entire third follower (unlikely), OR that he took over romance duties for Jennifer Hepler's companion.
As for Cullen and the need for a Templar perspective, I think Cass already kind of fills that role. Maybe it's not a perfect fit, like Cullen would be, but she's essentially coming from the same perspective; she's just more elite. I have a feeling Cullen will be a significant NPC, though, and that concept art you pointed out certainly solidifies that for me.
To go on a slight tangent for a second, I'd like to address something about Cullen. A lot of people assume that he shows up in that war-room concept art, but I don't think it is. Here's a post I made a week or so ago that didn't get a lot of attention (at least, not for the point I was trying to make):
"I'm going to attempt to make the argument that the guy that we see in the War Room concept art that we all thought was Cullen is actually someone else, and we might just know who.
My rationale is basically that the War Room concept art is probably really old at this point, and some of the characters featured in it may have gone through some major changes to their appearance.
For instance, this guy:

Has likely become this guy:

Both are elves, and both are wearing the same outfit, so chances are good they're the same person despite the other changes.
We know this second version is more recent because of Mark Darrah's update, which featured him:

So, what about this guy, our supposed-Cullen?

I think the chances are good that this is actually an early draft of our Ex-Grey Warden character!
Check out the similarity in their clothes (blue shirt with a big leather strap and shoulder armor):


So yeah, that's my hypothesis. I know it's pretty much pure speculation, but I feel like Cullen would probably be wearing his Templar armor in any concept art featuring him (such as that found in the old survey). "
Another thing I've picked up just from reading the forums for a while is that apparently Bioware is not big on giving three mage followers, since that would allow for an all-Mage party with a mage player character. That's why I went with the 4:3:2 ratio. I think four makes sense for the warrior class, since historically (DA:O) they made up the largest proportion of followers. IF Mr. Moustache turns out to be a follower, AND he's still got that ballin' trident, then I think he'll have to be a warrior with a non-standard moveset. Hard to tell at this point, that trident really could be a mage's staff and he'll end up some sort of warrior-mage hybrid, which would ALSO be awesome!
As far as romances are concerned, I'm kind of amping myself up to have maybe six romances. I know that sounds crazy, but ME2 had six romances with ten followers! With the DLC companion, six would be at least within the realm of possibility, although I'm certainly being optimistic. My current picks would be:
Cass, Viv, Sera, Mr. Moustache, Ex-Warden, and Iron Bull (please, please, that romance would be incredible). That's definitely all speculation, though, and doesn't include the possibility of Traynor/Cortez type romances.
I would comment on your Writer-Companion matching, but we're pretty much on the same page there (yes, I've definitely been trying to track that).
All I really have is that Patrick tweeted that his follower gets along with Sheryl's, but not (one of) Mary's. So this could mean Sheryl's only writing one.
Here's what I have:
DG (Cole?/Cass), JH (?), LK (Sera), MK (Var/Viv), SF (ScGirl?), PW (Iron Bull or Trident Guy?), SC (Grey Warden/Bald Elf?)
Meaningless speculation for the most part, hahaha, but it's a lot of fun. Enjoying this thread a lot, guys.