It would be great to have a functionality which allows to remove uncustomised Twig blocks from a template override. All blocks existing in the override template should be used from there, while the remaining ones should be used from the original template in the bundle.

This would further simplify customisation as you could do something like this.

Bundle template:

{% block content %}
    {# ... #}
    {{ block('additions') }}
{% endblock %}
{# ... #}
{% block additions %}{% endblock %}

Override:

{% block additions 'My addition' %}

So you do not need to copy whole templates anymore, but can just focus on what you need to change. This reduces maintaining the customised templates, too.

A similar functionality does already exist for translations where only a subset of all messages can be added to an override.