Ok pretty much Chaos gave us insight on vampires or demihumans. He said that they can act as terminals to other worlds, so basically, it is possible for Nagoriyuki to travel to another universe in Guilty Gear or could he intend on using Nagoriyuki again to bring him (Chaos) back into the reality? And could Chaos probably know about Slayer's existence?
Okay, you remember what happened during the Baptisma 13 Incident, right?
Valentine was "sublimating" (dismantling/killing) Gears in order to Open a Gateway to the Backyard.
That was (to her knowledge at least) the only way to enter the Backyard.
There's only so many ways to enter the Backyard:
1. Gateways from Gears (which could kill the Gear)
2. Pure Fluke (which has a low percentage of happening, like the woman in Izuna's origin story)
3. The Tome of Origin (which only recognizes Asuka R. Kreutz as the owner at the moment, not to mention is merged with him).
4. Spatial Displacement (swapping this world's space with the Backyards, like what the Cradle did over Babylon, which basically killed the entire population of Babylon in the process, not to mention created an Information Flare).
5. The Absolute World (which would kill all life as we know it)
The Tome of Origin is really the only "safe way" to Enter and control functions in the Backyard, and really that's the only way Asuka was able to create isolated spaces within that place.
Bedman "deleted his existence" without killing himself, which is (partly) how he created his Dream World (Nightmare Theater) which protects him from the Backyard's density itself, much like Asuka's "Isolated Space" (The Cube). Really it's a bubble that keeps whoever's inside it from being crushed by that information.
The Cradle is basically channeling the power of Justice "connected" to the Backyard's Magic energy output, which is why an Information Flare could be generated. Information Flares are basically "really BIG balls of Magic" that are so dense that they create a "hole" in this universe in to the Backyard. By swapping one Information Flare in the Backyard with another "space of information" in the Outside world... that's how the Cradle was able to teleport... but it killed a lot of people in the process.
BASED ON ALL OF THIS INFORMATION... it is NOT a "safe" or "easy" process for anyone to access a "terminal" to another Dimension/World.
Most likely, you'd have to kill a Demihuman (Youkai) to even get anywhere... if not otherwise "nearly kill" a Demihuman in the process (if Dr. Paradigm nearly killed himself just to get a "peek" at the Backyard, then Nagoriyuki probably wouldn't have much luck either).
The only people who would really have that kind of power are Izuna and Axl Low, and their Magic is unique, but has its own risks too.
If there does "exist" an item or object that could "create" a terminal in a "safe" manner, it either doesn't exist, or hasn't been discovered. So even something like the Tome of Origin has its own rules, and how it was created is still not quite clear in the first place... if something "like" the Tome of Origin existed for other worlds, no humans at the very least have knowledge of it.
SLAYER.... on the other hand, is an "exception".
He claimed to come from the Makai world, and while our first conjecture is that the Makai was a "layer" of the Backyard... it could in fact be another Dimension altogether, if what we know so far is true.
While Slayer said he planned on "retiring" to that world, he never actually said 'how' he would do such a thing... or if going back there was even possible for him (other than Zappa's joke ending).
Based on Slayer's conversations with Zato post-resurrection, however... it seems that the Underworld (where Zato's soul went when he died) does appear to exist... at the very least (though it may be similar to Izuna's Yomotsuhirasaka).
However, Izuna is still considered a Backyard Resident, and not a resident of any other Dimension.
If Slayer does have a "safe method" for entering the Makai (his home world), then it would make sense for other "safe methods" to also exist... but I don't think such knowledge would be easy to gain, for varying reasons.
Vampires keep a lot of secrets for their own protection, that much is a given.
Even the fact that Nagoriyuki's sword could be "harmful" to Gear Cells was a deep secret as well. How Happy Chaos got that knowledge is either a lucky guess, or he must've known something fundamental about the Flame of Corruption, otherwise.
Regardless, there's still not enough information to say how crossing over in to other Dimensions would work, let alone if the process would be a safe one.