What are things that are illegal today that could become legal in 50 years?

In the US? Child labor.

Depending on the state it's legal now. Arkansas...

In the USA, it'll become legal for corporations to fully own politicians and be treated as de facto nation states

Read Snow Crash
Why stop at owning politicians? Corporations already have a sort of Personhood. Just run the company itself for office.

Sexbots / Robotic Prostitution.

Might not be as advanced as in scifi, but...

Its more or less a perfect loophole for strip clubs.

Its not a person servicing you, you're simply paying for temporary use of mechanical equipment, like a gym.

This is illegal now?
- Maybe not strictly illegal, but ... as best I can tell, not really currently totally, explicitly legal.
  Kind of a gray area, I guess I mean that I think it will become more normalized and 'more' legal.

Many places are legalizing psychedelic mushrooms. MDMA is being used for veteran PTSD, and ibogane is gaining respect for severe addiction

I went to a weed store and they were selling shrooms and I was seriously caught off guard.
drug laws are going to be drastically different 50 years from now. EITHER people at power are going to be sensible (LOL yeah right) and make the beneficial ones less illegal OR everything will be illegal and testing positive for anything will get you the death penalty or sent to a torture camp.
We might see alcohol banned again, this time with the might of enshitification and modern technology to back it up. It's going to be a lot more ruthless and cutthroat next time around. Boy oh boy I can't wait for the absolute chaos that will unfold when companies start firing people left and right for testing positive for alcohol metabolites after they spend zillions of dollars figuring out how to make unbeatable alcohol drug tests with stupidly long detection windows instead of actually using the technology of modern medicine to do anything useful like cure cancer or help people.

Considering I'll be an old fecker in 50 years, I sure hope euthanasia is legal by then.

It is in some countries already.
- Some US states as well, at least it was a couple years ago. Oregon IIRC
SAME

Slavery.

While I get what you're saying, it is technically legal in the United States under an exception in the 13th amendment as punishment for a crime.
See more: https://en.wikipedia.org/wiki/Penal_labor_in_the_United_States
- Pedantry is the stinkiest of colognes, enjoy a space on my permanent blocklist
I don't know, slavery is pretty legal in a lot of places, including the USA.
Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.
Sounds pretty legal to me. Just gotta drum up charges against whoever you want to enslave in the USA.

Cannabis. Possibly psilocybin.

Not with the way the US is headed.
- There’s money to be made. Profit trumps “morals” in the US.
- In my state, it is already legal to buy supplies for growing mushrooms, possess an amount for personal use, and give and recieve them without payment. You just can't sell them for profit.

Probably literal slavery instead of the Quasi slavery we have now. They're just going to cut out the middle man I think.

I could see several forms of human genetic engineering becoming legal. For instance, three-parent children, also known as mitochondrial replacement therapy (MRT) is illegal in most western countries, though it was legalized in the UK, Spain, and Italy. The FDA banned in it in the US in 2015 because it involves human subjects research without prior approval from an institutional review board.

I would just be happy with being able to make eggs and sperm from stem cells so it doesn't matter what kind of pairing you have you can make babies if you really want to. Two ciswomen, two cismen with a surrogate, cisman and transwoman with a surrogate, ciswoman and transman, and so on.
- A lot of billionaires lately are concerned about population decline. At least, they claim they're concerned about population decline. But they're really ultimately motivated by the same "Great Replacement" conspiracy theories and eugenics ideals as any other right wing ghoul. I know for certain that Elon Musk is as much concerned about what kind of children are being born, rather than just that a certain number are born.
  I know this because, to my knowledge, none of these ghouls have ever directed any of their vast fortunes towards developing assisted reproduction technologies. These technologies include artificial gametes, but they would also include technologies like artificial uteri.
  Imagine being able to produce an embryo via in vitro fertilization and grow that embryo to full term without ever needing gestation in a uterus, a true artificial womb. Bypass the problem of infertility all together. Think what that would enable. With access to better and cheaper reproductive technology, then more LGBT couples could have children. And couples could have children at later years. Menopause would be no barrier to couples having children. The only limit on who and what ages people could have children would be based on what resources and remaining lifespan couples have to care for them.
  But Musk doesn't want to help trans people have children. Musk doesn't want to help couples have children in their forties or fifties. He doesn't want LGBT couples reproducing. He wants women at home in the kitchen, pregnant in their twenties instead of couples using artificial uteri to have children in their forties or fifties. He's far more concerned about his vision of social control and eugenics than he is about the impact of an aging population on national pension systems. If these billionaire ghouls were just concerned about an aging population, they would be pouring billions into assisted reproduction technologies. Instead they pour their money into politics. The fact that none of them have really shows a great deal about their motivations.

Brutal execution of people committing "crimes".

weed

Hunting the poor for sport.

Alternatively, hunting the rich for sport.

Look at Cyberpunk 2077... Take your pick

I will bet that a lot of genetic engineering restrictions in Europe will be more relaxed in 50 years than they are today.

The introduction of the car had people who were worried about it passing a lot of restrictions too; these went away over time.

https://en.wikipedia.org/wiki/Red_flag_traffic_laws

Red flag laws were laws in the United Kingdom and the United States enacted in the late 19th century, requiring drivers of early automobiles to take certain safety precautions, including waving a red flag in front of the vehicle as a warning.

In the United Kingdom, the law required self-propelled vehicles to be led by a pedestrian waving a red flag or carrying a lantern to warn bystanders of the vehicle's approach.
In particular the Locomotives Act 1865, also known as Red Flag Act, stated:
Firstly, at least three persons shall be employed to drive or conduct such locomotive, and if more than two waggons or carriages he attached thereto, an additional person shall be employed, who shall take charge of such waggons or carriages;
Secondly, one of such persons, while any locomotive is in motion, shall precede such locomotive on foot by not less than sixty yards, and shall carry a red flag constantly displayed, and shall warn the riders and drivers of horses of the approach of such locomotives, and shall signal the driver thereof when it shall be necessary to stop, and shall assist horses, and carriages drawn by horses, passing the same.
The Red Flag Act was repealed in 1896, by which time the internal combustion engine was well into its infancy.[1]

I was very confused at first. Here in Washington State "red flag" laws take people's guns away, which is very different than a person waving a literal red flag. xD
We like to outsource beta testing. /s

Bodily autonomy

In 50 years we're basically gonna be living in madmax. So everything is legal. Wild west baby

Hopefully, soft cheeses.

The mold that covers brie is dying out because we cloned it too much, we're gonna lose soft cheese.
- Welp, better a-brie-cciate it while it's still here.

Legal for who?

feel free to specify if you have something in mind

Punching Nazis

While I understand the sentiment, I hope you're wrong. Once you make it possible to legally punch people for any reason, it's really easy to just include more groups into the punchable ones.

Modifying a 4d hypercube to embed it with perpetual catharsis and spiking someone's drink with it

Violating laws.

I mean, as long as you're the POTUS it already is.