The Department of Justice revealed yesterday that Cohen himself has been under criminal investigation for months (details here).
Trump's lawyer is trying to claim everything seized is protected by attorney-client privilege, but the Justice Department has asserted, per a Reuters article yesterday (link here, mirror here), that an initial review of seized emails show that Cohen "is in fact performing little to no legal work."
A number of news reports on Cohen's actual work this week suggest he's really Trump's fixer, not his lawyer, and takes care of making messy problems go away, which includes paying people off to keep quiet and, at times, threatening them to shut up.
Given that Trump is Cohen's only client, what he's been doing for the President is obviously of interest to the Justice Department and criminal prosecutors in the Manhattan division of the U.S. Attorney's Office. (Special Counsel Robert Manafort is only tangentially involved with this particular matter and it's not part of his separate special criminal investigation into Trump and his associates.)
This paragraph in the linked Reuters article is of particular interest:
"Investigators were also looking into whether there was a broader pattern of tax fraud, wire fraud, tax evasion, money laundering and other crimes in Cohen’s private dealings, including his work for Trump and some real estate transactions that involved Russian buyers and prices that appeared to be well above market values."
Holy hell. This suggests that Cohen's files reveal the true smoking guns that could bring Trump to his knees. These would be crimes that have nothing to do with being President -- this would all be potential criminal activity involving his personal businesses.
Early next week, a federal judge will likely rule if any of the seized Cohen material is protected. Attorney-client privilege is not absolute and does not protect ongoing criminality involving both a client and the attorney. And, given the Justice Department's claim that Cohen isn't even really acting as Trump's attorney, but a co-conspirator who happens to have a law degree, the judge might allow everything to be seized.
This is something that definitely will need to be watched for new developments. Stay tuned.