I only just saw the cover for Volume 17 this morning, but that's not the point. The point is, last time when Volume 16 came out, people were noticing the similarities between it and the Volume 2 cover (Ed and Al in fighting stances). It made me wonder whether there was going to be a repeating pattern and had me hoping for a Briggs-tachi military-focused cover for Volume 17.
And yay, that's what we got! :D That still doesn't prove that there's going to be a repeating pattern, but let's just say that's what Arakawa is doing. Then what would the next two tankoubon covers look like?
The Volume 4 cover had Scar in the foreground with Lust, Envy, & Gluttony in the background. What I predict for Volume 18 is Scar in the foreground again, with Pride (his shadow form to keep from spoiling people) in the background, with maybe Kimbley along with him. Winry could be in there too, but I think she might look out of place? Also, I think the color theme will be a dull red, though purple could also be a possibility because that's Pride's color.
The Volume 5 cover had Winry and Den in the foreground. Likewise, I predict the Volume 19 cover will focus on Mei and Xiao Mei. :D Though I don't know what the story will be like in the chapters covered in that Volume, I think we can all agree that Rentanjutsu is becoming very important, and Mei only appeared off to the side on the Volume 14 cover. The color theme will be purple if Volume 18 isn't already; we haven't had a purple cover since Volume 11 so I just think there should be one very soon.
I dunno about Volume 20+, because it's hard to predict what the story will be like that far in the future. The manga might even be coming to an end at that point, so I wouldn't be surprised if the pattern breaks off there.
Now only to wait a few months to see how wrong I am. :D
EDIT: Also, some people say that the Northern arc has been moving slowly (at times I even thought so myself), but when you think about it, a LOT of stuff happened in just Chapters 66-69. Big plot and character revelations, action scenes, people getting slaughtered, etc. It only feels slow because we're reading the chapters monthly and Ed & Al are stuck in the fortress the whole time. When the whole arc is collected into tankoubon form, I bet you the pacing will be perfect.