If/when this season can't be finished, there will be lots of clubs across Europe that'll have reason to feel hard done by and look at taking legal action against the leagues.
A sensible option, which of course, wouldn't be without problems, could be the following...
Technically, the season would be null and void, however, scrap relegation, but honour promotion for the teams in the automatic places. This would increase the size of divisions, but there could be more relegation places at the end of the season.
As for European competition, the teams that qualified for it last season should compete in it again...with the addition of the teams that are currently in qualifying positions, thus expanding the Champions League and Europa League for a single season.
As I said, this doesn't solve every single problem and some teams will miss out on what they could possibly have achieved, but it would be a compromise that minimises potential for legal action.