Frankly there's no point speculating on what might or might not come out of any theoretical codex that may or may not even be released before 6th edition comes out, IMO. Judging by the 5th edition books that have been released so far we can expect to see at least 5 or 6 new units in the book and a huge number of Special Characters and options for the old units, but there's no way to know what those might be. That said, I'd expect at least some of the FW stuff to make it into any new book.

As to the rest:
Aun'va is the single worst thing in Warhammer 40k. The absolute last thing you want to do is make your entire army Stubborn, since all it does is increase the odds that combat lasts two turns instead of one and prevents you from shooting the enemy on your turn. The fact that he also comes with a(nother) crippling drawback and costs over 200 points for a unit that's utterly and completely useless in any offensive sense makes him pretty much the ideal way to ruin your army forever and guarantee that you lose every game.
Shadowsun is really quite bad. Her leadership bubble is okay, but she has basically no combat ability and again costs way, way too many points for what she does. She used to be worthwhile in Gunline lists, back when those were actually a viable concept. Now, she's really not worth considering.
O'shovah is amazing (free Bonding Knives alone means that he's actually about the same cost as a decently cheap Shas'el), as long as you don't mind not being able to take Kroot and being badly limited on Broadsides and Hammerheads, the single most important units in the entire book. Oh yeah... Basically, you take O'shovah if you're planning on spamming a lot of Crisis Suits and want to build a Deathstar unit of your very own. He's basically a niche character, and not really worth the tradeoff he comes with. Don't bother.
Yeah, our SCs span the range from "Worst thing in the entire game" to "Would be okay if he didn't have a crippling drawback". Lucky us.

Rail Rifles are not worth it. Frankly, the Markerlight is far more useful overall than the Rail Rifle could ever be and come on Pathfinders standard. Besides, the absolute last thing you want to do is encourage your opponent to shoot your Pathfinders. Disruption Pods give your vehicles a 4+ Cover Save against any shot from 12" or more away. They're pretty much the only thing keeping our vehicles semi-competitive right now, and are mandatory on everything except Piranhas. Seeker missiles are junk on anything except Piranhas (Turbo-boost behind an enemy vehicle then shoot the Missile at it. Most opponents do not expect this sort of thing), and even then should only be starting to show up at 2000 points and above as extra, "I've filled up all my Heavy Support with Railguns" anti-tank. Use your Markerlights to increase BS or ignore Cover Saves instead. I don't really know what you're saying with the last bit, but if I'm right about it, Broadsides shouldn't ever really bother to upgrade to Plasma Rifles, Devilfish shouldn't bother swapping out their Drones and Hammerheads should always use the dual Burst Cannons.