I am not sure what source they are planning to use. In this thread, we can give them suggestions.
In my view, it is fine if they use Uttara Ramacharita of Bhavabhuti so long what they show does not contradict Valmiki Ramayan. It is perfectly all right if they show something extra, but it should not be contradictory.
Valmiki Ramayan does not give description on the growth of all the kinds. But it mentions the names of all the kids. So, if Sagars show the growth of all the 8 kids, then it will be no contradiction to Valmiki Ramayan (it will be extra but not a contradiction). In that sense, it will be fine. In fact, it will be good if they show it.
Out of your views, I consider the conquests of Gandhara by Bharat, and Karapadhudesh by Lakshman, raj-abhisheks for all the 8 princes when they are crowned as very important because these are mentioned in Ramayan.
And in the end, they must show how Ram and his brothers end their incarnations.