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Well, I'll see what I can do, but it is very SF in nature...

To answer some of your questions, here....

It doesn't open any dimensional rift, really. FTL travel (that's some common shorthand for "faster than light") just isn't possible, as far as we can see. The mathematics for calculating the energy you'd need does weird things like divide by zero (not actually divide by zero, but similar things that happen in calculus and linear algebra) and falls apart. Essentially, time and length both go to infinity meaning that it takes forever to get nowhere, or they go to single points meaning it takes no time to get everywhere. @_@

Aside from that, though, Star Trek's warp drive also had the problem of inertia; what happens to the people aboard those ships when they are suddenly accelerated to five and six times the speed of light? They should be squashed into gooey bits of atom jelly. When someone pointed that out to the writers, they invented "inertial dampeners" to band-aid the problem. But they never talk about what those may be. There's a million other such little and lethal problems with warp drive which leads most die-hard Star Trek fans to surmise that the show takes place in a universe where physics is different and this stuff is possible. But, basically, the answer to the last question is "no."

Black holes are an interesting conundrum, though. They're objects that are so massive that even the speed of light isn't fast enough to safely travel past it. Yet all their mass is collapsed to a single point at its center of gravity, meaning that anything that was in orbit around it to begin with is just fine, since orbits are concerned with centers of gravity. Anything whose orbit decays, though, is going in. It will be ripped apart into a single line of atoms on the way in and then... we don't know. Some people have surmised that this matter gets completely converted from potential to kinetic energy and is the source of the x-ray streams that are emitted perpendicular to the accretion disk. Others have surmised that the black hole's center of gravity actually becomes a point-sized hole into another dimension and the matter passes into that, becoming the source material for a big-bang elsewhere. But no one really knows, for sure.

Getting back to fantasy, though....

Magic portals between places in the same world can be a lot of fun to work with. I figure that each particular brand of magic will have to come up with its own explanation for how they work, though. In my world, it's completely an Astral Plain thing; you momentarily hop to the Astral Plain and that ferries you to the exit.

I wonder what would happen if a portal were set up to exit on the very spot you entered. Would you run into yourself and not be able to get through or would that be impossible because you haven't entered the portal yet? Hmm... I could have some fun with this...

Berz.