This is not a concept for a Cursed Technique Reversal, but rather an experiment to invert the core concept of a Technique. Shrine in itself seems to be inspired by cooking, and the name gives away some aspects of his character and the nature of the technique. A Shrine is a divine location in which offerings are well... Offered up to a deity. Sukuna himself was previous deified. Alongside that, he's described as a natural disaster. He cuts down anyone in his path of destruction in order to live his life. While I intend to invert the technique, I'll keep some of the themes (and such) such as divinity.

Shrine → Sanctum...

A Sanctum could be considered the opposite of a Shrine whilst keeping a theme of the divine. Whilst a Shrine demands sacrifices and offerings, a Sanctum functions as a holy place where you are offered various things such as food and shelter.
Now, getting into more AU or "joke" territory, an inverse of Sukuna wouldn't be "The Fallen" or some sort of Imaginary Demon. But rather, some sort of Benevolent Deity. Basically like those memes where Sukuna is some sort of kindly spirit.
Instead of Cooking, which is a final destructive act that renders something into food, Sanctum would be themed around preservation and the act of cultivating something. In a way, the process of growing and caring for something could be considered the opposite of preparing and cooking it, even if animals raised to be made into a meal are technically just being prepared.

Dismantle → Varnish...

Dismantle is an all purpose slash which is stated to basically be the default. A knife that simply fillets something with a fixed amount of force. So in an Inverted Technique, the most basic, all-encompassing form of preservation would be to protect. Thusly, the opposite would be a coating of Cursed Energy he can apply to any object or being. This could stop the effects of bleeding or maybe even reattach limbs if you want to push it.
Assuming Inverse Sukuna is a purely benevolent "deity" or perhaps even some form of Bodhisattva, he would use this technique to cure people of their cuts and wounds. In battle, he would use it to amp up how much Cursed Energy he can use to reinforce himself. Effectively, this application allows the user to coat anything in a layer of Cursed Energy. Beyond that, he could add layers upon layers of reinforcement onto an object or person. So he'd be a physical menace, relying on Varnish as a form of armour to fight in close quarters without fear of ever being harmed.
By the way, the direct name of this skill was inspired by an older post where someone made a Technique Reversal for every Technique in JJK.

Cleave → Coat...

Instead of applying a small amount, Coat acts as the same way as Cleave. A Technique which adjust the amount of Cursed Energy used. If Infinity is a technical defense like a magical barrier, Coat would be like a thick layer of armour. Think Senator Armstrong's Nanomachines, but with Cursed Energy instead. Effectively, Coat streamlines the process of combat by utilizing the amount of Cursed Energy he imbued into the Technique in order to automatically reinforce moments before an impact. We could even see him using Coat to reduce the cost of RCT. If Varnish merely tints him in his own Cursed Energy, Coat would consume him in a flaring aura of it.
When used on an attack or weapon, Coat would also Reinforce the weapon or attack. Like an automatic system that imbues his strikes with Cursed Energy, doing so just enough to break through an enemy's defense. I know now it just sounds like I double-dipped on Cleave, but the whole concept was preservation and the technique so far has worked via reinforcement.
This could also act as a form of imprisonment, binding a foe up in Sukuna's own Cursed Energy in order to prevent them from moving as the Cursed Energy that isn't theirs wouldn't have to default to protecting them or harming them.

OPEN : Furnace → OPEN : Wellspring...

Furnace represents either pure destruction or the finality of cooking something, as chemical bonds are permanently changed by the process. You can't exactly un-cook a meal. Wellspring acts as the opposite, a form of storage where you keep water. While furnaces are used to cook and heat stuff up, wells are used to store things. Namely water and salts were most commonly kept in wells. Wellsprings can also refer to naturally occurring abundances.
Wellspring would act as the opposite of Furnace. A strong opening move, a set up in which Cursed Energy is made to bloom outwards like the waves in a bucket when you fill it up. It scatters and imbues the environment with Inverse Sukuna's Cursed Energy or whatever the equivalent would be in an Inverted World. As we see with Gojo's demonstration to Yuji, to a degree, you can manipulate objects you've poured your Cursed Energy into, even without a Technique.
This move would reinforce the terrain and put inanimate objects under his control. Perhaps in the Inverted World, he used this to build infrastructure like the truly benevolent soul he is.

Domain Expansion : Malevolent Shrine → Domain Expansion : Benevolent Sanctuary...

The Inverse of Sukuna's Domain. It is an closed off domain which has the Sure Hit Effect of Coat & Varnish on any level in which Sukuna desires, even to a detriment, burying someone under the weight of countless Coats and Varnishes. This acts as a way for Sukuna to restrain them non-fatally, as he would be precise enough in his control of his Domain to be able to pick and choose who is affected akin to Yuta Okkotsu and Yuji Itadori (the latter of which was able to install a "no violence" rule until he felt like triggering the Sure Hit).

Cursed Tool : Kamutoke → Cursed Tool : Uchimizu...

Kamutoke is derived from the term or concept of Kamitoke, or "to be smited/struck by lightning" which explains exactly what Kamutoke's ability is. To strike someone with lightning. Everything points to the core concept being judgement or just straight up annihilation.
Taking the form of a Ceremonial Water Vessel (Yi), Uchimizu is an item capable of creating endless torrents of water with various properties. The water can be drinkable, brackish or salty, acting as a vessel that turns Cursed Energy into water. Uchimizu is a giving blessing (or flood disaster but hush) from a deity whilst Kamutoke would be the LTG level judgement.

Cursed Tool : Hiten → Cursed Tool : Daichi...

I'm gonna be completely honest, we still don't know what it does. My favourite concept for Hiten is the one on a now missing or deleted post titled something along the lines of "The Itching Scar of the Kamo Clan" which I swear I'm not crazy. But from what I remember, upon being commanded to fly with a target in mind, Hiten would pierce through space in order to go the fastest route towards the target. A spear that specifically seeks out the user's target. And no, it wouldn't bypass Infinity. It cuts through space to find the shortest route, effectively taking the path of least resistance. But it is still taking a path and moving through space.
Hiten means Flying Heaven, which is why I've picked the name Daichi, which roughly means "Great Earth" as in the land itself. Ironically, I would like to think that Inverse Sukuna would be more proficient in his usage of Cursed Tools since his technique is not offensively inclined.
The opposite of a spear is a hammer. A spear is a sharp weapon designed to kill. But what about warhammers?! Don't care. Hammers are blunt tools which can be weapons, but are primarily used for building, crafting and so on. Daichi is a short hammer with the ability to move the Earth. Instead of moving through space or the air (most of the time as a thrown or flying spear would), Daichi manipulates the Earth with every strike. Carving and raising various points to the user's own command. It can be used to disrupt an opponent's footing, outright attack them, kick up dirt and so on. It's very versatile.

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