Well, since he hasn’t been indicted, its all secondhand speculation as to what crime he might be charged with, based on what is publicly leaked, which may not accurately or completely represent the results of the investigation.
That said, the main crime that is expected in the NY case is bookkeeping fraud for the false way the hush money payment was recorded in business records as a legal retainer, possibly with an enhancement related to that fraud being used to cover up another crime (one of the speculations in that regard is that it will be charged as covering up a federal criminal violation of campaign finance law.)
> What is the crime worthy of arresting a president?
Neither the US in general nor the State of New York have a legal principal that being a former President at the time charges are filed creates a higher standard for criminal charges, or arrest based on those charges, than exist for any other person subject to the law. There are countries that have that kind of legal lifetime privilege for certain officials or other persons, but not the US.
Yeah. Usually longer... or never. We should be glad for what few we can get.