I didn't want to make the sketch a "complicated" feature, since I think that a possible full fledged infinite whiteboard should be a separate feature or a thing for a piece of code outside Telegram, like the games.

An example I want to make is that we can share stickers on Telegram, but we don't have a full fledged image editor.

The limited size of the canvas and the limited choice of brushes keeps the message light in size and easily shareable / editable.
Since the canvas won't be sent as a raster image, I agree that it should be possible to implement it with features like editing and remixing, like you suggested.

I also wanted to keep this feature accessible even to users that aren't artists, so they can be more "leveled" with professional artists and use the feature more often without be ashamed.