Well, over the history of the country, we haven't had to decide an issue like this because, heretofore, presidents have been mostly law-abiding.
So, if the country were to somehow stick to something like the course of the last 250 years, the repercussions would be limited to Trump escaping criminal liability, if it were to be decided that he has immunity of some sort from criminal prosecution.
Of course, if another lawless asshole were to be elected, s/he'd be more free to do crimes in office without fear of repercussion outside of political considerations. Again, if there is some degree of immunity for actions in office.
On the other hand, if the president is found to have little or no immunity, that won't change much except perhaps to slightly constrain executive decisionmaking.
We've had decisions like this before, mostly post-Nixon, resuming a bit with Clinton. Because the country has stayed the course for the last 50 years, these types of decisions are not significant on a daily or even decennial basis. But they are pretty fundamental to the operation of the country.