It should be possible to have an extension interface that allows a spellchecker/grammar checker to get a button on text input fields such that if you click it,

It should be possible to have an extension interface that allows a spellchecker/grammar checker to get a button on text input fields such that if you click it, then the extension is activated on that field at that time. That seems like a much better Least Privilege design to me than giving the extension access to... literally everything you do.