nuvoleweb
Use Drupal's theme keys inheritance
Long story short.
\Drupal\ui_patterns\Definition\PatternDefinition::PATTERN_PREFIX should be "pattern__" instead of "pattern_" to allow to make leverage of the theme keys inheritance.
It would allow to use a theme_preprocess_pattern that would apply on all patterns.
If you don't want people to change their template names to "pattern--my-pattern.html.twig" you can still force the template name in the hook_theme() call but I think it should be the new default (that you can make overridable in the pattern definition to keep future compatibility for PatternLab and Fractal).
This has quite some implications, also related to work that @aleksip is doing on derivatives. I considered doing this at the beginning but then I realized that each pattern is really a different theme function, just prefixed by pattern_ for namespacing sake.
We can still add a global HOOK_preprocess_pattern() preprocess by using theme suggestions.
I'm not sure that it is going to have the same result.
I can't find the time right now to make some tests but it could really be interesting because I believe that when using pattern__foo theme key, both hook_preprocess_pattern and hook_preprocess_pattern__foo are called, unlike when using template suggestions.
Do you know if I'm right or would you have some time to try this out?