What's one big change you'd make to Sadako to make her truly viable or closer to that?

Make Condemnation actually threatening so that survivors have more incentive to get rid of it, and therefore not be on gens for a bit

She’s actually already probably as good or maybe just slightly better than Wraith so she may not need a big buff. But if her stats turn out to be too low and the devs buff her some possible ones might be

• Remove her lullaby. I’m not sure why a stealth killer has a lullaby in the first place? Maybe they’re worried that her being 115% and almost always Undetectable would be too strong? Like I said, she may not even need a buff so that thinking might be right, but if not removing the Lullaby would be helpful.
• Lower the TV refresh timers. The faster the TVs come back the more often she can teleport where she wants which improves her map pressure and her frequency that she inflicts Condemned.
• Make all survivors who have Condemned build it up passively over time similar to when they are carrying a tape. Maybe if you have a tape it passively builds up more quickly than if you don’t, but either way having the Condemned build up on its own once you have a stack would increase the pressure survivors have to deal with it.

Sadako isn't in as bad a spot as some people seem to think, but there are a few things that I think could be done to help solidify her strengths without taking her overboard.

Mostly, her TV timers should be shorter, that's genuinely the biggest one. 100 seconds is a looooong time in this game, I definitely think that could be a fair bit shorter.

I'd also like to see the TVs changed to face the generator more reliably, but that's a minor thing, it just bugs me when I play her.

Slugging actually improves your pressure, especially with her because of her high map mobility. By slugging you are trading securing a hook state for an additional 10 seconds or so getting into a second chase. So if you can hook one specific survivor when you down them and slug the others for example then you are keeping typically three people off gens and also focusing your carrying time on just one person to eliminate them. Plus because of all the extra teleports you get doing this you have the side benefit of building up stacks more quickly which also adds pressure and sometimes even a kill.

Depends on how much you want to slug versus hook. For extra slugging though Jolt and Infectious Fright are both good since Jolt gets you extra slowdown per down without needing a hook or kick and Infectious Fright tells you immediately if the next survivor you want to chase is nearby or if you’ll need to teleport to a gen farther out.

Also having a perk to help in chases is useful since, like you said earlier, once she’s in a chase that’s a weak point. So Save the Best for Last or Bamboozle for instance are both good depending on whether you want to go for followups after injures with STBFL or if you want to shut down very strong windows at certain loops. You could also try the Enduring plus Spirit Fury combo since part of a slugging strategy is playing the long game and forcing a lot of pallet drops to create dead zones for the mid and late part of the match.

If you only want to slug very occasionally then Scourge Hook Pain Resonance and Dead Man’s Switch is a decent combo since you get the regression from the hook and can then immediately teleport to the gen that it pops to force it to be blocked. Plus with her teleporting Dead Man’s Switch has extra value in general since she can pressure multiple gens in rapid succession. You can also try Make Your Choice if you plan to hook more since you can often leave a gen quickly with her, then when a rescue happens usually use a nearby TV to try for a potential Exposed hit.

In either case a lot of people would also recommend Corrupt Intervention as a way to slow down the early game to give you more time to get those early downs and pallet breaks you want for the long game. If you really want to focus quite a lot on early game pressure you could try Lethal Pursuer, although keep in mind that her TVs won’t be on until after Lethal Pursuer ends so you’ll be doing a standard Stealth ambush on someone with it instead. Likewise for Whispers to help get an early hit or down faster, plus Whispers has an added benefit with her teleport that if you teleport somewhere and see no Whisper ping you immediately know to move elsewhere.

As far as add-ons go, especially if you’re slugging, the Green add-on that adds stacks to survivors who heal each other is huge. And that red add on that refreshes TV is also great. If you don’t happen to have the red add-on then you could maybe use the add-ons that reduce the TV refresh timers to get a little bit of that same effect, or possibly the add-on that speeds up the Condemned timer on people with tapes if you’re worried about someone using tapes to shut down TVs. (The goal being not so much to kill the person with the tape directly but to keep them off gens a bit more.)

Regardless of the build though, I think the main things with Sadako are

• She’s weakest in an extended chase, you definitely want to lean in to spreading pressure on multiple survivors. So having a solid macro game of knowing where everybody is and which survivors to chase over others and so on is really important. If a survivor drops a pallet, take it as a win that you get to remove the resource and move on to pressuring someone else off a gen unless you think you have an opening to follow up quickly on the pallet dropper.
• Just because she’s Undetectable doesn’t mean they can’t see her coming. Take advantage of her short height to help her approach survivors from behind obstacles where they’re less likely to see her, and try not to always look directly at gens when you approach them to mitigate Spine Chill’s impact.
• Prioritize keeping track of Boons and snuffing them when possible. Like all ambush killers Circle of Healing is somewhat strong against her if left unchecked. Fortunately once you know where they place a Boon the first time survivors tend to repeat placing Boons in the same spot so you’ll have a pretty good idea where to look when they replace it or run off to heal. And with the TVs there’s a good chance a TV is fairly near whatever Boon they place, maybe even within hearing distance.
• It’s a small thing but her partial invisibility after Manifesting can help her mindgame at loops. Head one direction around a loop while Manifesting, then as soon as they lose line of sight backtrack. There’s a decent chance you will still be invisible while backtracking and the survivor will run right into you.
• Obviously Unbreakable is a thing but since it only works once per match it’s actually not as impactful as you might think when you slug the same person multiple times. But as above watch out for Boons since Exponential might give them multiple free revives, and if you are running any Hexes keep in mind they might have Soulguard. (And of course No Mither survivors you may just want to hook out versus slug. But honestly if someone runs No Mither that’s usually good for you regardless. 😄)
• Lastly when I say “slugging” I don’t mean necessarily going full throttle for a four person slug win. The idea is more prioritizing your hooks where you focus hooks on one survivor and mostly slug the others. And once you get one person eliminated then you have more time to capitalize on hooks more freely, especially if in the preceding part of the match you’ve eliminated a bunch of pallets to make the late game chases much easier.

condemn gives other effect instead of a quick mori. Like she gain more invisibility and some debuffs on based on how many stacks.

i don’t know what is the point of her actual ability you go semi invisible but still have a lullaby and can’t attack. Personally I would like to see a actual invisible killer like give her complete invisibility and no flickering crap. She already can’t attack so it can be used to actually patrol gens without being uav scanned every second by swf

Make survivors carrying a tape suffer from the Oblivious status effect until it's deposited. And remove her unnecessary lullaby.

this

1. Remove her lullaby. I've had much more success at throwing on monitor and abuse and just not demanifesting save for teleports than "stealthing" with her lullaby. Stealth and lullaby do not go hand in hand for obvious reasons. Or significantly decrease the lullaby range. 24 meters is too much. By the time I approach a gen that I heard being worked on, half the time the survivor has already made it behind a tree. Her lullaby is essentially a 5th perk for all survivors who aren't already running spine chill. Because they know you're coming before you even knew you were going. Go figure.
2. However, the biggest rework needed is for condemned. Tapes are completely a joke right now and often serve as more of a BM distraction than not. I understand that a free mori shouldn't be easy to achieve, but a killer's main ability shouldn't be completely and utterly useless and unused in a majority of games. A condemned survivor who gained their final stack of condemned through holding a tape should be punished. A survivor in chase should not be able to pop a tape in your face "as counterplay". In fact, as soon as they gain their final stack of condemned from holding a tape, they should become Exposed and unable to insert the tape for a full minute. "But the survivors at 4 stacks of condemned won't be able to strategically remove tvs." Good. Why is game balance for this killer alone centered around the survivor's ability to control the killer? The survivor will have to choose a generator near an already powered down tv or accept the risk that Sadako might teleport to them. Or they will assume the risk of becoming fully condemned and have to hide for a minute. Do something to make condemned not entirely meaningless. Making it to 4 stacks should be something to be avoided, not something easily remedied at your own delight.
   1. A survivor who gained their last stack from Sadako teleporting is not (entirely) a Survivor mistake so perhaps you could punish that less severely. Sadako should see condemned stacks as not knowing that information makes it difficult to determine when condemned was gained legitimately or if its just being used as a free alternative to Diversion. Otherwise, wholesale replace it. Perhaps, allow Sadako to see condemned stacks as base kit, and make an add on where teleporting near survivors gives her current ability of adding one stack.
3. Reduce her TV cooldowns. In full SWF sweat lobbies, you're only going to get about 3 uses MAX out of a single TV before all 5 gens have popped. Don't buff it where her iri is unnecessary, but where it is now its more safe for survivors to literally ignore TVs and gen rush. Give them a reason to turn TVs off and assume the risk of grabbing a tape.