Perhaps the most hazardous aspect of nexus points is the likelihood of there being an active dimensional rift in such a place. Rifts are not merely doorways -- they are rips in the fabric of space-time itself, and who can predict what might occur?
01 A demon, supernatural intelligence, malevolent god, or other monstrous entity is attempting to emerge from the rift, but is stuck or otherwise unable to break through fully. Perhaps it is out of phase with our reality, or ancient wards placed on the nexus point might prevent its crossing the threshold, or perhaps it is simply too large to fit. At any rate, it is displeased, and will lash out (physically, psychically, magically, or otherwise) at anything in view or reach.
02 Shifters have cast a powerful enemy through the rift and are trying to close it before they can re-enter our world. GM's choice whether the Shifters are good or bad, and whether who- or whatever it was they're trying to seal off deserved it.
03 A seasoned, well-equipped Coalition force has known about this rift for some time, and has set up a sort of base nearby. They will attack anything emerging from the rift, and will likely do the same to anyone trying to enter or otherwise interact with it.
04 This malformed rift constantly expels weird mutagenic energies into the surrounding area. Anyone exposing themselves to it must roll for strange effects. GMs, pick your favorite mutation or insanity tables (or both).
06 Raw magical energy spews forth from this rift at certain times -- if there is a pattern, no one has yet been able to discern it. At any given time, there is a 20% chance that a bolt of pure magical energy will surge forth, granting whomever it strikes 5d100 points of P.P.E. (determine target randomly). However, there are potential side effects, including (at the GM's option) physical damage, insanity, mutation, etc.
08 A stable rift that leads to the world of another role-playing game (or other fictional property, if you're feeling daring), possibly beyond the Megaverse itself. Pick something from your collection.
09 This rift leads to a "mirror universe" version of Rifts Earth, where bad is good (baby) and vice versa. 100% chance of the player characters encountering bizarro versions of themselves.
10 Not technically a rift, but a dimensional vortex that actually sucks in and swallows bits of the surrounding reality until some sort of threshold point is reached, at which point it vomits forth chunks of another dimension entirely. There is a high likelihood of this extradimensional binge and purge having gone on for some time, which means the surrounding environs will be a jigsaw puzzle of otherworldly elements.