I see. This might be surprising to hear, but the essence of this is technically already possible in 0ad. It is possible in scenario scripts to push basic decision dialogs to the player and then wait and react differently depending on what the player chose. Now, the system isn't optimised for showing a lot of dialogs in a single game, as the code can get quite cumbersome, but it is surprisingly versatile and works. The reason it's so unknown is because it isn't actually used in any playable scenarios right now, there's only a small demonstration of it in the "Triggers Demo" map (the yes-no dialog).
Sooner or later, I'd like to write a full guide about embedding storylines in scenarios and campaigns, to show that features like this exist and how to use them.