In general, will the possibility of a romantic relationship with someone other than companions be realized?
Notice that between two characters non origin as companions, it is also not possible ...
At this stage, I think unfortunatly 0 %. It would request a lot of scripting and dialogs to be efficient just to get something generic and BG3 is not supposed to replace the Sims.
Perhaps a mod would be created by a fantastic modder but I am not optimistic about it.
