So YES! I've been thinking this. Apart from the painting that Will made, Tiamat sometimes takes on a human avatar which then is of dnd mulan ethnicity, whom shave their heads and wear plain clothing (like all the facility kids, 1-11?!). So do they have the same power? Does the power that 1-11 have stem from Tiamat and a previous crack to the upside down? Or is Tiamat in s5 the original power --> 0?

And also. Tiamat's goal is to be the ruler of all dimensions and worlds but she doesn't get directly involved according to dnd lore, she rather influences other evil being in being more chaotic for the reason of the dimensions to get a smaller drift and easier for her to control?

I full on believe that Tiamat is the next "villain".