I know it’s kinda a late response, but I’ve just searched for the topic of creating notion widgets by yourself and I came across your not answered question.

The below answer is kind of nerdy, if you want to create your own widgets, you have to learn HTML, CSS and JS first, then you could try to host your code, optimally with GitHub Pages.

The thing is that I am starting to develop my own notion widgets and I am on the step of designing them.

You asked about the framework of such a quest - it is completely possible to achieve with pure HTML, CSS and JS, but the important part is about hosting your code.

You could host your widgets completely free with GitHub Pages from your own repo and embed them into Notion, with only one restriction - your code has to accessible to the public (the code repo has to be public).

I am going to extend apart from pure HTML, CSS and JS with Astro.js, TailwindCSS, Firebase (database), and potentially some other little pieces of technologies.