Support for Jinja's Block Assignments would be nice.
These allow writing {% set <var_name> %} blocks that store the block-content to the variable name:

{% set navigation %}
    <li><a href="/">Index</a>
    <li><a href="/{{downloads_url}}">Downloads</a>
{% endset %}

Filters applied to the variable name will be applied to the block’s content:

{% set reply | wordwrap %}
    You wrote:
    {{ message }}
{% endset %}

My idea of implementing this would be somewhere along the lines of generating:

{
    let mut __askama_writer = String::new();
    // ... block-content as rust code
    __askama_writer
}