When the devs will start Rework the b777-300ER?
It should begin within the next year since they are reworking the entire 777 family.
Short answer: We don’t know.
Long answer: We don’t know what variant is coming next, nor when.
Only the dev team knows this. We have to be patient and wait.
We don’t even know what 777 is coming next, I would suspect the next ones are 777F and 777-200LR and the 777-300ER will be the last one to come
Maybe we already have? Maybe not? Who knows these days (: