I just recently noticed this because I was interested in making a psyker. Is there no advanced classes to go into or other powers to choose from in Only War?

Well, they get their own chapter in the book, some bits in another book that has nothing to do with them, their own weapon type, and they are supposed to get their own AC book with Stormies, Commies, and them, but we still don't know if that will happen, because FFG likes to play things close to the chest, we still don't know if the OW line is being continued, or if they are walking over to the DH2 table, and ignoring what they've already squeezed most of the money they expect to get out of it series behind. My opinion, of course.

Sort of smacks like Naruto CCG. Like the game, or hate it, when the last set got made, theyhyped it up, like all the rest, and made no hints that this was it. We were primed to get the first of several more broken, OP characters as cards, and things were getting good, and then they announced that it was done. They gave everyone a chance to buy numerous packs, and get the stuff they wanted, and THEN said we're done. No more contests, no more cards, yada yada, and if you bought into the last set, thanks for the money. Now, from a business standpoint, I can see this, as manufacturing the cards is an expense, as is distributing them, and if you sell them as the last set, you won't sell many, but it still hurt to find that we were done, and Bandai would move onto other card games.

I don't know if FFG is quite the same, but they don't seem to want to say they are done with OW, nor sate our hunger with news of any upcoming products; the typical publishing slap in the face that I hate in most of these scenarios, where they don't often seem to care about you, but they do like your money. All that whining said, I do hope that they pop out the last book of the trilogy o' advanced classes, even if I have some trouble imaging what good ACs they might get, and what bonuses they'll bring. The Psyker already has magic, and can raise their PR as high as they want. Short of several new good powers, several new shite powers, and a few psyker talents, I'm not sure how they'd improve the career, as a whole. Again, that's just me, and that's why I trust others to write my role-playing books, for the most part.

I refer you to my stopgap House Rules entry - "The Tempest" - in the appropriate board. It's not official but it's something.

Psykers are so horrendously OP compared to the other specialties that they don't need anything else, really. Besides, when everyone else is advanced specializing, the Psyker can just get a free +5 to WP for dedicating to their specialty.

Which is very good for them. Sort of like I said, I'm not too sure what perks an AC would get them that trumps +5 WP.

They could give them a big ass book of powers, some fun non bolt stuff(putting delayed psyker bombs in people you possess, I dunno) or some generally super OP variations on what they already have (PRd10 damage for PR times 10 meters). A new slightly different force weapon. Some more rituals, maybe something that messes with the enemy psykers divination or telepathy. An AC that's more focused on melee. Maybe just a super powered psyker alternate class that's just straight up better.

They don't need their own book, but giving players another can of worms to open at the table is always fun.

They could give them a big ass book of powers, some fun non bolt stuff(putting delayed psyker bombs in people you possess, I dunno) or some generally super OP variations on what they already have (PRd10 damage for PR times 10 meters). A new slightly different force weapon. Some more rituals, maybe something that messes with the enemy psykers divination or telepathy. An AC that's more focused on melee. Maybe just a super powered psyker alternate class that's just straight up better.

They don't need their own book, but giving players another can of worms to open at the table is always fun.

I think the reason Psykers, Stormtroopers, and Commissars didn't get their own book is for the reason you outlined above - most of it is just overpowering an already overpowered character. There is a reason Sanctioned Psykers are a rare sight on the battlefield (and don't really belong as an option - but never mind that) in that they are extremely powerful. Why should a player become even more dangerous than the rest of the people at the table?

Well, for fun of course! All the other gamelines ended up feeding psykers more and more abilities as it wore on. Some of them quite over the top, and while they never needed it, that's the nature of splatbooks. A bunch of cool stuff, a lot of it OP or poorly stated, some of it awesome. It's always fun to play with new guns and swords and vehicles. For the tables that do allow psykers it's fun to play with new moves.

How your table deals with psykers being more powerful is up to the table. I'm mostly okay with it, and I in generally dislike intentionally unbalancing certain characters. Though for whatever reason having psykers, Space Marines, and normal people in the same group doesn't bother me as a GM or a player that much. I like how it fits with the setting, the other characters can still kill their fill in combats. To me it's more of an issue of is everyone at the table okay with it, and I haven't personally had any gripping, But I definitely know why it rankles many. Especially in an OW game where the PCs are supposed to be bog standard grunts "Why the hell do we get our own witch?" Different strokes. I personally get a kick out of checking out new disciplines, even for gamelines I'm not running, or when I'm not trying to stat up a new psyker NPC.