Thanks for the feedback.
With “Widgets” I really meant just like any kind of custom PHP code that would, once the theme is active on the site, pull data from the database.
I said “Widgets” because I saw Blocs offering Widgets from WP in the sidebars as “Data source”, but also other WP Data (site name, etc). So I refer to the generic dynamic data to populate a certain bric in Blocs.
For this, I wanted to use not only “inbuilt” widgets and settings, but produce a set of custom options/widgets/brics (not sure how to call this, I think dynamic data for Brics is correct?)
So we could populate a certain bric with data from ShortCode, or literally any PHP function that gets data from the WP Database/API.
The problem with Gutenberg, @brechtryckaert, is that it is not ready for realistic development, neither in the current “page” builder mode nor in the in-dev FSE experience.
Several performance issues, usability issues and a very blurry line drawn in the FSE implementation about who does what and where, unfortunately lead to a solution which may be good for some quick drafts and experiments but not a serious development that is based on sustainability, neither performance (energy) wise nor maintenance wise. At least, not within my experiences. I give it always a new try, and always end up moving away back to good old simple but stable HTML/CSS/JS/PHP.
Too many times I created something in blocks and the next bigger update, because the thing is simply still under development (even if added to core WP) things changed or where lost. That, together with the massive hunger for resources the editor has (Check for example the taxonomy selector in a post editor, compared to pre-gutenberg), has convinced me that I need to wait a few more years to really sell that to clients 
Well, and that is also a reason I am checking out apps like Blocs, which BTW after a night working out on it starts to make me like it.
Now about this thread here, I understand it is not yet possible, but perhaps will in future, which would be of course super cool.
Thanks for all inputs!