Setting custom block.html.twig templates in preprocess function (not based on twig naming scheme)
I managed to get it working by using

function mytheme_theme($existing,$type,$theme,$path){
        return [
                'wide_grid_block' =&gt; [
                        'path' =&gt; drupal_get_path('theme','mytheme').'/templates',
                        'template' =&gt; 'wide-grid-block',
        'render element' =&gt; 'elements',
        'preprocess functions' =&gt; ['template_preprocess','template_preprocess_block','comment_preprocess_block','mytheme_preprocess_block'],
                ],
        ];
}

function mytheme_theme_suggestions_block_alter(array &amp;$suggestions, array $variables) {
if($variables['elements']['#configuration']['id'] == 'page_title' || $variables['elements']['#configuration']['id'] == 'product_selector'){
        $suggestions[] = 'wide_grid_block';
}