I'd really like to see a campaign/setting book with all the alien fauna and flora you can shake a stick at, might as well throw in the mercenary races too, I like the sound of them

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Throw in an Arachnid mutation upgrade sheet, updates on how the war of the Species is progressing (Been reading alot of Battletech recently and the method used by them for their scenario books would work really well)and sneak peaks of things we might see in the future and everybody's happy.
Maybe have some kind of competition to get fans to write a couple of articles/races for inclusion (Like the crab people) to raise interest, and honestly I dont see where you can go wrong.
Throughout all the books, I'd like to see more fluff and more consistant artwork. Take a page from Rifts and have awesome artwork, fleshed out with fluff vaguely linked to it.
I agree with Nanite that having it all from a Sicon perspecitve gets old pretty quick, a couple of storys told from the skinnies point of view, rebel colonists or even races not activly engaged in the war of the species all seem an obvious way of breaking up the monotany (And despite what Hiro seemed to be saying, its entirly possible to write from an aliens perspective without using their fictional language, Im not sure how anyone could even make that claim to be honest...).
A decent page count is also a must as well, things like Armybooks are normally the worst perpetrators (the Arachnid army book is easily one of the worst gaming purchases I've made), if its the choice between putting out a book where its an actual struggle to meet the minimum page count and not putting out anything... well sometimes tough choices have to be made.
Given all that, and the past releases, I've gotta say Im in favour of one big book a year, even if around 40% of it is reprinting unit stats from S&P.
Im not sure if Mongoose made much money off the army books (Or the nasty splitting the MI into 4 or 5 books...) but given the price even a pamphlet sized book ends up going for these days a single £25 book is alot more tempting (Even when it ends up going for sale in the US for $30), it'll get more use in the long run, and big pretty books with lots of pictures and fun content do ALOT more for a system than five 40 page ones.