Michael Dorf talks about the "corruption premium" that Trump is at risk of taking advantage of and touches upon the constitutional provision against emoluments. More here. As suggested in the latter link, Congress might provide some safeguards there, including giving judicial standing to challenge. I also would offer disclosure rules and the possibility of relief being connected merely to business in question (such as a perk obtained). Note there is not a total ban; "without the Consent of the Congress." Impeachment is not the only remedy.