I'm also wondering about that myself and can't contain my excitment for the future chapters.
For me personally, I think Shiemi might be one of the demons who had help the exorcist in the past, the emperor of creation, Shemihaza. Their names are quite similar and the image they used to describe Shemihaza was of plants which made me think of Nee and shiemi who has a great affinity with creating plants etc.
And then let's not forget the fact that the Grigori agency was also the place where they had one of the other emperors, Azazel, hold up there too. Considering the chaos that is starting it was for the best to bring her back considering her state.
This is just my theory on what Shiemi might be but I also like your idea that she might be an angel or a descendant etc. because then we will have angels in play for what's to come in the story.
But besides all this, I'm even curious about the connection between Amaimon and Shiemi? Mephisto even knew who she was and didn't tell Amaimon. The look on Amaimon's face in chapter 99.2 when he heard Mephisto kinda made me feel sad for him.
That's true, she does look like them and they all called the other person Mother
What is Shiemi?
Personal Opinion Spoiler
The organization that came to get her is the Grigori Agency, and Grigori are Wacher Angels, so she could be a descendant of these angels. Also on Wikipedia (A very trusted website(●￣(ｴ)￣●)) They list some of the Grigori as Azazel and Samiel. So, she could possibly be like Shiro was and a descendant/product of the research that was in the institution. There was also the Naphalem in the meeting in a previous chapter that could be the possible "Mother" of shiemi.
I really want to know what Shiemi is!!!!!!!!!!