Well the headline issue will be I suspect Americas withdrawal from NATO, ending the status quo which has kept the world free of nuclear conflict for a few decades now.
How about withdrawal from the Paris climate accord? You probably agree (assuming you don't think climate change is a conspiracy theory, if you do then stop reading here), that that is a bad thing for us all. In addition he has already (in his previous term) removed a number of emissions limits on US industries, causing further environmental damage as a result.
Other issues, oh I don't know, America is already edging towards a Handmaids Tale situation with the introduction of controls/bans on abortion, building walls along borders, insurrection in the Capital, and whilst that is a domestic matter, it still does concern me as a supporter of freedom of choice. (And please don't throw back that I'm advocating against that by supporting the removal of Trump by any means necessary, sometimes the end justifies the means)
As an aside, as President he extolled the killings of the leader of IS and at least 1 leading military figure in a sovereign nation (Iran). If he supports assassination of political adversaries on the international stage, why the outrage when he too is the subject of a politically motivated assassination.