Tinkerer might rearrange these rows in the book of girls as they get more crowded. I doubt that they are written in stone. He could also decide to remove the place holder in the center row, where the goblins are right now in order to make more room for future LIs (if it is a place holder and not the portrait frame for someone very special). In general I think the current arrangement in the book of girls isn't set and open to rearrangements in the future as it seems fit to Tinkerer.
- I also think that the MCs mom will be one of the future LIs. She's already in the game anyway as a ghost. It might be possible to summon her in the future or even revive her. We'll see, but I'm pretty sure, that she will be another LI.
- After the amazing Brigitte update, I'm also very sure, that people will want to have Siv added, too, as the last missing giantess.
- Daiyu makes people curious since her introduction as a saleswoman. So she will be in that list at some point, too.
- Cath so far is a mystery, she was only mentioned in a letter as the one pulling the strings. I really hope, that she will be added to the game as another LI and so do others.
Hence I'm pretty sure about those 4 mentioned above. (Mc's mom, Siv, Daiyu and Cath)
Apart from these there is talk about a female Orc and a female dwarf at some point. Personally I think there are more than enough goblin women already, but there is a demand for them, so Tinkerer might even add one more like a cave goblin.
Qetesh and Nergal are interesting end game choices. After all who doesn't want to have sex with the goddess of fertility?
For the rest, this is pure speculation with very little evidence.