Fetch all unused assets? How could I go about fetching all assets that are not currently beeing used anywhere?
With good help from @carlcs here and on Slack, we found a solution:

{% set allEntryIds = craft.entries.limit(null).ids() %}

{% set usedAssets = craft.assets({
    relatedTo: allEntryIds,
    limit: null,
}).ids() %}
{% set unusedAssets = craft.assets({
    id: 'and, not ' ~ usedAssets|join(', not '),
    limit: null
}) %}