Zhu Jiu says is Shen Wei can't maintain his full form in Haixing for long, and Shen Wei himself says he can't use his full powers on the surface. So maybe that's connected! Maybe the portal is meant to give them an appearance that will fit in, and a "reduced powers" version of themselves that can be maintained under surface conditions?

That's right! Yeah, I've wondered what Shen Wei in his "full form" would be, because I didn't get the sense that Zhu Jiu simply meant his Hei Pao Shi form, and Hei Pao Shi in Dixing doesn't seem different from Hei Pao Shi on Haixing.

I really like the idea that the portal gives them an appearance that will fit in. In setting up the portal, I think it would make a lot of sense for that kind of traveling between worlds.