Mass deportation.
I feel like people expecting the guardrails to impede Trump in a second term haven't really been paying enough attention to what Trump and his allies have been doing.
Like yes, mass deporting 20 million people would require an immense government force and would likely include deporting legal residents. But they most of all are well aware of that.
The first Trump term wasn't just bluster. He intended to do a lot of terrible things. Incompetence got in his way in a lot of cases. In others, key individuals in certain positions developed moral backbones and refused.
For the second Trump term, Trump intends to revive Schedule F to fill the government with vetted ideological allies. And Trump's allies have developed plans to circumvent institutional barriers.
The irony here is that Obama deported far more people than Trump, Biden is on pace to match Trump, and over the past 60 years, Democrats have deported more people than Republicans. It's hard to take either side seriously when it comes to deportation and immigration issues in general because the rhetoric does not match the reality on either side. If you want to vote for who deports fewer people then you can't go wrong by voting Republican. There's 60 years of data to back that up, including Trump's first term.
Congress determines the laws for denaturalization within the confines of the Constitution and previous Supreme Court rulings, not the president. Generally, denaturalization can only occur if citizenship was obtained via fraud and the process has to go through the courts. Trump can certainly pursue more cases but a judge (and in some cases a jury) has to make the decision in each case. There were more people denaturalized under Trump's first term but we are talking in the neighborhood of 100 people per year vs. the more typical number of around a dozen people per year. I sincerely doubt these were good people we should feel sorry for and whose rights were violated but admittedly, I don't know the details of those cases.