Dren and Fang are the only ones so far who have triggerable dreamscapes, contract-wise Xora, Bunaby and Yikshi have them too. However, if you want to count the valentines event as well, then all of them except for Sanma have a dreamscape already!
Your character needs to be in the aroused state when you talk to them in their room. Well, Bunaby and Yikshi technically have a dreamscape in their contract scene!
Oh, don't worry about it! yeah, for the valentines, Sanma, Yikshi and Bunaby don't have one. Ah, I thought I got every bug in Dren's and Fang's dreamscape. I'll be fixing that as soon as possible!
