I don’t think that this will be some cop-out making Sienna a Necromancer. I would consider it even more likely getting Undead as new enemy faction in some kind of Estelian campaign (which purely by chance has also been overrun by Skaven during the End Times).
Can also serve as ramp up for Sienna’s career change to live a better life and why she might join the Knights of the Blazing Sun.
My guess would be a melee only career.
This would allow for a completely new gameplay with two magic melee weapons and a magic ability, that could perhaps serve as ranged kind of like Bardin’s crank gun.
Otherwise, her ranged options are all fire staves and she wouldn’t feel different enough to her existing careers, which is an objective FS seem to be looking for in these premium careers.
Double ranged with one weapon having melee like the javelin. Maybe one of her passives could grant her a shield that blocks 1/2 attacks every x seconds to compensate for the lack of blocking.
Two-handed flame sword - “flamberg” - is definitely needed. Most importantly, there is a modern miniature with such a weapon.
Please god no, no more melee-only careers
Ranged-only career I’m open to
Trollhammer torpedo can block. Could easily add a staff that blocks, available on that career only. Not even a problem for alt. firing mode, since weapon special button exists.
I’m much more fond for a melee only Sienna than a Ranged Only Career, and I hope that if they put a Melee only Sienna they rework her weapons to each have a Special Attack that generate/cost Heat
They will most likely just add 1 or 2 weapons that do that. Or just make her overheat work differently on that career.
I think some kind of stealthy class is missing from Sienna. A smokey, soot wizard who uses smoke to hide her from the enemies and stealth about unnoticed while being able to choke enemies in a similar way to hagbane or a poison cloud does. Give her dual daggers, 1h mace or even something like dual wield maces would be quite good fun.
Her ULT would be akin to the other stealth classes being able to hide from sight, reposition and cause damage from stealth.
Thing is, if they allow any of her current ranged weapons on her new career she will be limited to being another fire wizard ; and I doubt they will release her with more than 2 weapons.
If she’s not a fire wizard, her melee options will be limited to 1h Sword, and her ranged options to nothing.
Either she’s yet another fire wizard with a different gimmick (as @Argonaut14 pointed out, stealth would be a yet unexplored one) ; or she’s some other kind of wizard (which would fit the overarching theme of complete character overhaul though the RP explanation would be a tad ass-pulled).
In that case, I don’t know how they’re going to justify why a Golden wizard has a fireball or a beam that burns nor a dagger or a flaming flail.
Most of her staffs could be used for other type of magic, they just need to change the particles on the staff and problem solved.
I’d like a lightning wizard.
Why would they apply a burn DoT though ?
Lightning causes burn injuries.
If it’s not desired for some reason it can be renamed to lightning DOT.
Turning her old staves into another Wind would as much work as making them new, maybe even more because you don’t start from brand new foundation
The more I think about new career options for Sienna, the less I’m convinced that the developers are as constrained by her current careers as people think they are.
The Warhammer Fantasy Roleplay Career Compendium (2009) explained the game’s career system thusly:
As a character advances over the course of a WFRP campaign, the careers he adopts become a sort of journal of his life. The story behind the Rat Catcher who seizes an opportunity to strike it rich by becoming a Thief could be wholly different from the Rat Catcher who takes up arms and chooses to become a Shieldbreaker. Or the Apprentice Wizard who eschews the other wizardly careers to instead focus on acadaemia and becomes a Scholar. The countless other career combinations also have their own unique stories to tell.
Sienna’s next career doesn’t even have to be any kind of magic-user. A character doesn’t completely abandon or suddenly forget their old abilities when they choose a new path in life. Sienna would simply continue to make use of Aqshy in addition to the tools and tricks of her new trade.
You can argue that choosing a non-caster career and continuing to use magic would make her a hedge wizard and therefore a target for the Witch Hunters and the Colleges of Magic, but that already applies to her Pyromancer and Unchained careers. Only Battle Wizard represents a Sienna who has completed her training and is thus an official member of the Bright Order.
That said, if the recent Lohner’s Chronicle entry really is setting up Sofia Fuegonasus as a sort of dark mirror for Sienna that she must confront again, one of the most fitting options for Sienna’s premium career is the Master Vigilant mentioned in the OP. A career for stealth, espionage, deception, and other dirty tricks up her sleeve. Since master vigilants rarely operate openly, the writers could even try to pull off the twist that Sienna has secretly been a master vigilant all this time. Saltzpyre’s reaction alone would be worth it.
That convinced me. That would be an absolutely beautiful turnover. Not likely, maybe, but beautiful. 
Never really considered it before.
All that said, Sienna’s Careers are heavily ranged focused so far, with more interesting ranged than melee options, and creating a class that can simply not use them at all would just be weird.
On the other hand, Unchained is pretty melee-oriented: She doesn’t have the ranged bonus the other two have, can get a massive melee power bonus, is tanky as hell…
Things Sienna doesn’t have so far:
What she definitely does have in every career is complete independence from ammo drops and some class-specific interaction with the overheat - BW gets rid of it faster and could choose to channel faster, Pyro get’s crits, unchained gets melee power. It is certainly possible, but I doubt they’d just get rid of that for her fourth class.
I do like that the Vigilant suggestion(I believe that was on reddit a while ago, as well?) potentially ticks several of those boxes, namely finesse, stealth, and… actually I suppose also support.
It’s funny you should mention Sienna’s independence from ammo drops, because I was thinking… Both Bardin and Kerillian now have access to overheat mechanics, but what if Sienna moved in the other direction? As the internal investigators of the Colleges of Magic, master vigilants are dedicated to rooting out students of dark magic, necromancy, and diabolism. You know who often practice these arts? Vampires.
Fire didn’t seem to work, so maybe it’s time to put Sofia Fuegonasus to the ground with silver bullets.
This is all very good, but the visual should come first - because this is Warhammer. Remember - that I liked Zealot career because it was based on cult classic Mordheim miniature, as well as the Bounty Hunter was based literally on the cover Brunner The Bounty Hunter omnibus.
On the other hand, the KoBS is an iconic image known to everyone. Besides the female knight is especially good, recently there has only been a lovely new miniature of the female foot knight.
Sure, I got no argument against that. One of the reasons why I want a Warrior Priest of Sigmar – specifically of Sigmar – is because the hammer guys are one of the most iconic things about Warhammer ever, maybe second only to Slayer dwarfs when it comes to iconic visual appearances.
My Sienna suggestions are a bit tongue in cheek anyway… Although I have to admit I’d love it if Sienna went full Bloodborne with her premium career, with a silvered blade in hand and silver bullets for her gun.
Not a necromancer Arthadaw, not a necromancer. 