Agree on a Naval Planetary Auxiliaries book. It'd cover all sorts of air support units. Not just the much needed Valkyries, but also the other atmospheric combat vehicles - fighters, bombers, etc. Could quite easily enivision something like a pararescue unit fitting right in.
Agree on a tanks/armor/vehicle book. There's way too little coverage of how to run a tanker game, but lots of vehicles to use in one.
And - a book that includes a great deal of coverage on command of a unit, and more details on how to deal with/handle/approach things when you're not just a random squad of low-level grunts on your own.
I would second the vehicles book. Sections on "how to fight a tank" (Which I've posted extensively on so I won't repeat it here!) followed by "Air combat and Close air support" (It ain't always as fun as it sounds!) And maybe even a little information on non-combat transports (Which are not even addressed in TT!). If you really got ambitious, you could even put the rules for Titans in there! (They are sort of a vehicle :0 )
I'm not sure about Titans. I mean, sure they're iconic, but they're also on such a different scale that makes smoothly integrating them as anything other than mostly background elements difficult (in Only War they seem to want us to be infantry grunts, unless you've got a psyker or are being artillery spotters, there's basically no way you're going to do much more than scratch the paint on a Titan, and it's not going to have to try to kill you). On the flip side, Deathwatch did give us the Warhound scout titan, sooo......
Nonetheless, just as it would be interesting to have a campaign built around being air crews, it would also be interesting to have a campaign built around being a Titan's crew. Might be a bit like Rogue Trader in some respects.