Imperial Navy I'd say should be a seperate book all by itself (the "Battlefleet Koronus" of OW, if you like), and additional archetypes beyond the ones that have already been presented could be great for OW's answer to the Inquisitor's Handbook/Into the Storm.
I do, however, agree with what some people have been saying that Valkyries should absolutely be in the core book if drop-troopers are, though the problem with that is that it would require an entire new section on aerial combat and such, so it might even be better to strip drop-troopers out of the core book and hold them back until OW's IH/ItS alike comes out. Perhaps the space that is freed up by doing so (one space opens up in "regiment type" and not having the Elysian Drop Troopers sample regiment frees up an entire page) could be spent adding in rules for roughriders as people have suggested. As they've pointed out, hunting lances are already in, so why not the people who would use them? Rules for horses, Tauros and Venators couldn't possibly take up more than a page. Oh and the core needs listings for how much ammo various vehicles can carry for their weapons, but I've talked enough about core already in this thread ostensibly about stuff that should come AFTER core…
Probably one of the obvious ones is a bestiary-style book. The OW bestiary should provide GMs with infomation about each of the 'main' enemy 40K armies (Tau, Eldar, Dark Eldar, Chaos, Necrons, Orks and Tyranids?) operates and how they're organised in addition to simply listing what forces they can deploy. It ought to give GMs ideas about how each enemy behaves as an overall fighting force, which will in turn help them determine what kinds of forces the players might realistically end up facing in a given part of the warzone.
I've always had a bit of an issue with Elysians being in core without their main vehicles, Valkyries and the Tauros. That's just not right; I'd rather wait to see them done properly than have half their rules.
Horses and Bikes would be lovely, seeing as Hunting Lances are in. Rough Riders could be a fun Specialization.
I think that OW is keeping to Chaos/Renegades, Orks, and Dark Eldar as their enemies. Though the Guard fights all of those other things, 'Nids and Necrons will tear through them like a hot chainsword through some skull-butter.
My apologies to anyone I offend; FFG staff, playtesters, and forum users alike.
Please check out my Dark Heresy to Only War conversion! You can find it on the main Only War forum. I'm always looking for more people to playtest it!