Long story short:
-Fluff about not-humans
Fun fact. In every single picture that contains more than one army, one of the armies will invariably be human.
I'd vote for a third option, an extra helping of xenos, not a deluge of them.
Long story long:
Xeno-Fluff: Here's the problem: The entire game is written from the standpoint of humans, always has been, probably always will be. And to be certain its not a bad thing. Even in the codices of other armies it is often written from the outside, from the human perspective. Now should they include more fluff info on those other races? Sure! But it will necessarily be based largely on the human element - if for no other reason than all of us reading it are human.
However I don't think that shouldn't devalue those little vignettes narrated by say, an Eldar about to kill some Orks or fork over the booty to some daemon. But for instance they can't really write from Tyranid perspective very well. It would end up sounding entirely too much like cookie monster goes to war, "Om nom nom *stab!* om nom nom *sneak* om nom nom *shoot!*" etc.
- Battlefield pictures: Aces! More please. The more dynamic miniature set-ups the better. They're inspiring both to the gamer in me and the painter/hobbyist, and are usually awesome.
- Missions: I presume you mean rules for missions. I've been playing off and on since the early 90's, and no matter what missions have been included, most people I have come across just want to play a stand up fight. Often because the other missions are poorly designed, unbalanced, or stupid. Not all but every editions has more than a fair number of stinker missions.
- Battle Reports: this is an absolute impossibility. You might wishlist it, but anything other than the most generic game (e.g. not a game worth reading about) comes dangerously close to creating dissonance between the main rulebook and the codices. During any given edition what, about half the codices get rewritten, maybe more? Which threatens to lock the writers into certain models, fluff, rules, etc. How strange would it be if the battle report in the rulebook followed one set of paradigms but a year later that same army plays significantly differently?
Battle reports are a non-starter from a corporate marketing perspective. And that is also not a bad thing. By not having any long form/written battle reports the authors can focus on the rules, the fluff, and showcasing models. Sure models change and get converted. But that is a lot less confusing than a 'conversion' of the rules. A new player who reads the rule book and sees a battle report with an army playing out in a certain way SHOULD be able to expect that is how the army plays - if it has been rewritten in the interim, that is likely not the case. Besides battle reports are better off in white dwarf anyway; being periodical is the perfect way to produce that sort of narrative tied to a particular rules set at a specific time without unduly tying the writers to it.