What I have seen are reports that he wants to annex Greenland, Canada, and and the Panama Canal...as well as change the Gulf of Mexico to the Gulf of America. Not sure how true those reports are.
You're seen headlines, you mean. Or maybe very selectively edited clips on networks who then have hyperbolic talking heads clutching pearls and going to hypothetical extremes.
I'll break down each of these 4 for you based on what he's saying, his intent, and the reality.
Trump genuinely seems interested in acquiring Greenland. He's said Denmark's rights to it are disputed, and the people there would vote to join, and it'd be in their best interest. He's expressed willingness to make a great deal with Denmark for it, or threatened (very unlikely) to sanction them. His interest in Greenland is due to a massive build up of Chinese and Russian naval aggression, especially given China's influence within Canada. Greenland would be used as a military deterrent. This is good for America, Canada, and Europe. Chances of happening? 50%.
Canada? Trump is trolling. He has no real intention of making Canada part of the US, though, I think he's open to it, it's not a true goal of his. He's basically forcing Canada to better trade deals after we've been getting screwed by them for years. One reason Trudeau resigned was because he's been so owned by Trump since the election he knew he was a dead man, so he's saving face. Like Greenland, if Canada wanted to join the US, he might consider it, but there's a huge difference between making a continent with 45,000 people Americans, and making a country with a population of 43 million part of it.
The Panama canal Trump is 100% all in on. He WILL take it back, and it's the ONLY one where the military might possibly be used. We built it, we maintained it, and we've become weak on it, and China has made major inroads in controlling it, and thus, major inroads into influence in our hemisphere with the potential to greatly weaken us. Trump won't rule out military use here, because while there's a 5% chance of that happening he wants China and Panama to fear it IS POSSIBLE. Why should he or would he show his cards and deny he'd use the military? But make no mistake....Trump IS serious about taking it back, and he will. Much of the Greenland talk is likely meant to worry China. You're expanding? two can play that game.
As for the Gulf of America, it's a troll, but even if he did, who cares?