This has been suggested quite a few times in the past. But, there might be some problems.
What if a team wins both the Cup and the Champions League; participation in 2 Super Cups is not appropriate, but would it be fair if a single cup and a cup-double winner are rewarded the same way (participation in Super Cup) in this context?
Cups winners have to be chosen level-wise to play against CL winners, but participants in Cups are frequently not at the same game level.
So, for there to be a super cup, there must be a cup competition similar to that of the Europa League, putting the situation where a team cannot win both cups that give access to another competition.