"What can Donald Trump do that badly in 4 years?"
A lot actually, but oh congress won't anything pass right? Wrong, Republicans now have the majority in the house and the senate. Trump is also responsible for those members of congress getting re-elected.
EPA and FDA will be gone. Roe v. Wade and Marriage Rights are gone. Obamacare is being replaced by "something tremendous". We'll have the Bush Tax Cuts on steroids. Republicans are about to start tearing shit up.
First, love the avatar.
Second, I wouldn't be too worried about this right off for several reasons. First being that the Democrats will have at least 48 Senators in Congress. That is important because there is something known as the silent filibuster which means that it would take 60 votes to bring up a bill for a vote. If the Democrats don't agree with one of the bills, they can block it from coming up. When Barack Obama was elected, shortly after Ted Kennedy died and the Democrats only had 59 votes. Even though the Republicans only had 41 Senators, they successfully blocked a majority of Obama's legislation. This can happen here too. I would assume especially with Bernie Sanders, Elizabeth Warren, Tammy Duckworth, Al Franken, Tim Kaine and Cory Booker and many others, we will probably see some pretty strong filibuster attempts.
Third, Trump gets to replace Scalia, the most reliable conservative vote on the court. This means the court will be no different than it was when abortion rights have been upheld and when gay marriage passed. Trump would then need one of the liberal justices to die, since they have indicated they have no plans on retiring. Yes, they are getting up there in years, but that doesn't mean they can't live through a Trump 4 year presidency.
Fourth, even if Trump does get to replace more justices, it doesn't guarantee anything will be overturned. There is a concept in the law known as stare decisis. When the Supreme Court decides on a law, it sets a legal precedent. VERY RARELY will the Supreme Court revisit a case to overturn a precedent. It has happened, but it is not only rare it never happens very soon after a precedent is set. It would undermine the Supreme Court to make a decision on gay marriage and then a few years later, just reverse it. The likelihood of this happening is very small. Roe vs Wade, that I would be a little bit more concerned about but I think we should wait and see what happens. If the Democrats take back the Congress in the midterms, which tends to always happen during a president's first term, we can block his nominations just as easily as the Republicans did with Merrick Garland.
The EPA and FDA are completely safe. Trump cannot eliminate them without an act of Congress. Most Republicans even do not agree with Trump that they should go. He doesn't have the vote to do that even if he wanted to. If he ended up getting Republican votes, the Democrats would block it.
One thing I want everyone to keep in mind is that we are making the assumption that Trump is going to play nice with Republicans anyway. They spent the entire primary trying everything to keep him from winning. They then all abandoned him after the Access Hollywood tape came out. I'd be surprised if he just starts getting along with them now. Trump has a tendency to say what he thinks is necessary to get what he wants, but when he gets it, he also reverts. At the moment we don't know how many of these policy decisions and these court justice picks he was serious about.
Now, what Trump can do and probably will do is withdraw from NAFTA. He will likely impose a high tariff on trade with China. This would be bad.
But the main thing to think is that this is not the end of the world. If he had a filibuster proof majority in Congress, you should be concerned. What we are likely to get is four years of gridlock. The democrats are going to do to Trump what the Republicans did to Obama. They have the numbers that they can do this. Our government was deliberately set up in a way to make it very difficult to pass anything. There are lots more checks and balances and even with one party controlling both branches, they doesn't make it easier. For example, Obamacare passed the Senate before Ted Kennedy died, when Democrats had 60 seats, a filibuster proof majority. Once Kennedy died and was replaced by a Republican, they 41 Republicans prevented the 59 Democrats from passing another health care bill. Obamacare was not supposed to be a final bill. It was supposed to be a blueprint that would be adjusted. However, since the Republicans didn't allow them to do anything, the Democratic House of Representatives passed the unfinalized bill that the Senate had already passed and the Affordable Healthcare Act was sent to Obama.
My point is that everything is not gloom and doom. More likely than not, Trump is going to end up being a very ineffective president. There are the possibilities that bad things might happen, but many of them are unlikely. We need to take it a day at a time and see what unfolds.