Some of the things the Democrats may investigate in no particular order
President Trump’s tax returns
Trump family businesses — and whether they comply with the Constitution's emoluments clause, including the Chinese trademark grant to the Trump Organization
Trump's dealings with Russia, including the president's preparation for his meeting with Vladimir Putin
The payment to Stephanie Clifford — a.k.a. Stormy Daniels
James Comey's firing
Trump's firing of U.S. attorneys
Trump's proposed transgender ban for the military
Treasury Secretary Steven Mnuchin's business dealings
White House staff's personal email use
Cabinet secretary travel, office expenses, and other misused perks
Discussion of classified information at Mar-a-Lago
Jared Kushner's ethics law compliance
Dismissal of members of the EPA board of scientific counselors
The travel ban
Family separation policy
Hurricane response in Puerto Rico
Election security and hacking attempts
White House security clearances