I have some great ideas what makecode arcade can update and add on it’s editor, but I don’t know how to create extensions. If it is possible, then, what language should I use?
Here’s the ‘getting started’ docs on extensions: https://makecode.com/extensions/getting-started.
I would suggest using GitHub to store / distribute any extensions you make: https://makecode.com/extensions/github-authoring (though I would note that there were significant improvements to the github integration after the last release, so you might want to wait a little bit for those to be released first)
Here’s an example one (my status bar extension): https://github.com/jwunderl/pxt-status-bar.
And finally, you’ll want to use typescript to author the blocks.
If you have any questions feel free to ask! There are lots of options / etc with blocks and it’s very possible we’re missing documentation. The easiest way when you’re starting is usually to just find a block that looks like the one you wanna make, find where it’s created (e.g. sprite.say here: https://github.com/microsoft/pxt-common-packages/blob/master/libs/game/sprite.ts#L461), and base your block off that. You can also use https://makecode.com/playground to quickly test out the shape of the blocks you’re making (whatever you paste on the left side will be injected as blocks on the right side when you press the run button)