The Vessel - Playing with Portals

I only got a small glimpse at the idea of a Vessel class, but I had a player over the New Year try out one of the options: the Animancer. Their choice was largely due to the obvious power of having teleportation portals.

As you'd expect out of magic portals, the possibilities are nearly endless; they can be used for so much more than just getting from point A to point B. But portals are fun, and thinking about using portals is fun (with or without cake). So, rather than remove the option entirely, I want to restrict them in order to prevent the power from being a "too easy" solution to basically every problem. On one hand, rewarding creative use of one's powers is exactly the type of thing I want out of my callings. But on the other, the key to that fun is limiting those powers in reasonable and interesting ways.

My first thought was that I could limit the portals to "ritual" speed - essentially preventing their use in action scenes. While such a restriction certainly is reasonable, it is not interesting. Doing this sucks nearly all of the fun out of being an Animancer Vessel (assuming that portals are your "go-to" thing).

One issue with the physics of magic portals is when they change the world's angles, as in, going from a hole in the ground to a whole in a side wall. Flippy floppy worlds can be fun to think about, but are nightmarish to declare rules for. The core of that problem is also the problem with having floor or ceiling portals at all: the acceleration of objects moving through those portals to ridiculous speeds. So, while I hate to simply sidestep design issues, this seems pretty straight forward: we could only allow our rifts in space-time to exist parallel to gravity.

But this of course is now not very reasonable. Your current question is most likely, "what about in space?" And you're absolutely right. However, this is where we are going to allow the setting and memory-style magic to wibbly-wobbly-timey-whimy explain it away. Basically, we'll say that portals are inherently very unstable (edits to space-time seems like they should be), and that if put at an angle, the gravitational force of one side of the portal would pull on the other, and much like a stretched spring, snap back into place, quickly balancing the two sides until they are both aligned. If the force of the gravitational differences is too great, the portal will simply be torn apart. Either way, only portals completely parallel to gravity will be stable enough to be functional.

So far that's all I've got. What kinds of fun (or broken) things can you do with these portals? What kinds of other limitations could we put on portals, so that they are interesting RPG tools, yet no so easy as to solve all problems?