If not "Treason, Bribery, or other High crimes and Misdemeanors," what is this? That is the wording used in Article II, Section 8 of the United States constitution, to wit:
The President, Vice President and all Civil Officers of the United States, shall be removed from Office on Impeachment for and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors.
Dating back to English common law, impeachment was, in the words of the annotation used for the foregoing link, "a device that figured in the plans proposed to the [Constitutional] Convention from the first, and the arguments went to such questions as what body was to try impeachments and what grounds were to be stated as warranting impeachment." The citation, from a 1919 law review article, notes the "catch all" nature of the concept - separating out "other high Crimes or Misdemeanors" from "Treason" and "Bribery" - but arguably, in the current situation, with the 45th sitting President of the United States, even "Bribery" may indeed play a role in the basis of charge (think of the "hush money" payments made at Trump's direction).
Regardless, there is no carve-out for an "indictable offense" - something that, according to prevailing Department of Justice policy, caused the Mueller investigation to fall short of indicting or even recommending indictment, as it otherwise would have been a pointless act. Instead, it is the House of Representatives that indicts, not the Department of Justice. As a pre-emininent, co-equal branch of the federal government, it need not wait for a secondary agency or authority to say whether it concludes that indictment or impeachment is appropriate. When the facts warrant it, which they clearly do in Trump's case, the House must probe for the existence of grounds to indict and if finding the grounds, move forward with an impeachment proceeding, to be argued before the Senate. All that need be shown is the malfeasance of the office holder is of such "high" character that under the circumstances, there is a gap between the fair and reasonable expectations for the office and the gross misdeeds of its current holder that demands political correction.
In this context, there can be little doubt that Donald J. Trump has indeed engaged in deeds that are grounds for impeachment, and his right to hold the office of the Presidency must now be held to account. To do anything less, if you are a current member of the United States House of Representatives, is to do a disservice to your country.