Cotni Cooks

Fog:

One of many potential ways to make the gimmick more aesthetically complete (rather than just a sudden shift with no explanation) would be to add a projection map overlay such as a vignette, that only appears when close to the portal
one way of making the vignette only appear when close to the portal would be: to have it be the only material in the track affected by fog (this means you are using a skybox that always draws regardless of Z in XLU pass for the projection map overlay, as opposed to adding it as another layer on many other materials)
then set up blend settings and fog colours such that the fog usually causes the overlay to become invisible (for example, fog makes it black, and it uses additive blending)
then use a BFG area with a transition time near the portal that pushes the fog away or changes its colour.

Mipmaps:
A better solution might be to use a stencil alpha texture on the portal that is opaque in far mipmaps but can fade into holes up close; this is more of a gameplay visibility one than a worldbuilding cohesion one like what I thought of.