Well, one of the central ideas used to be, admittedly, that people could go through universes, but nowadays it's the fact people are stuck on the Core Timeline and all of the king's horses and all the king's men and even all of the so-called smartest and/or most powerful people in the multiverse haven't been able to solve the problem (so far). Thinking, blasting, inventing, hacking -- none of it has worked.
Dunno whether or not to add something like people being able to have two-way communicating across universes (something like the Elminster could still work as one-way "pigeons" being sent sound kind of cool (and kind of jerk-butt, too, but alleged gods can be like that)).
I can see alternate planes having Earth facsimiles of their own, or even full-blown parallel Earths having appeared on the Line (maybe that "counter-Earth" that shows up on some sci-fi stories, and Earths on other solar systems).
As for the barrier, it's supposed to be that anything Bug that tries to cross it to get out dies. Arachnids, viruses, creep, whatever -- same as the viruses on Raccoon City. Things can enter, but the monsters can't leave (I guess it's possible for other Kerrigans to send "pigeons" in to strengthen the forces and the Kerrigan on Hawaii has some other... insidious.. methods to contact out (like, I guess, psionically possessing some guy to transmit out to Korpulu).
The idea was there initially because while other people can say "oh, we could just drop the Sentry there for some bug-hunting" or whatever, I didn't saw it as anything else but a doomsday scenario. The field prevents that hell from spreading throughout the whole planet, but it definitely allows that one place to be a perfectly good representation of Hell on Earth.
I guess it still means there's ways for people to be infected with the Viruses or to get Arachnids (maybe they get samples that make genetic data models that they use to "3D print" the bugs? That's one way for one Kerrigan to send stuff to another, I think).
I don't know. We could discuss this detail. In general is about making it fun, in-universe and out -- they are a threat, but they are not an instantly apocalyptic threat... otherwise it would be all about dealing with the Queen B of the 'Verse and the Ashfords wanting to become gods 24/7 and everybody jumping on their bandwagon.
As for SHINRA having a base on Syracuse... maybe the Mako Reactor is not complete. There was some... successful, and pretty energetic... let's call it "protesting" and the Reactor was never completed. There's nobody sucking the Earth's Lifestream dry to power up his washing machine.
But the metaphysical damage, so to speak, is already done. The place is plagued with the constant appearance of monsters of the entire Squareenix catalogue. Anything from Moogles up to Heartless.
One of the reasons SHINRA has gotten the goodwill it needs to continue to function (on the Line, at least) is Materia. They are very good at finding, mining, refining and mass-producing it, and I guess as well as stuff like that mecha you mentioned they also are good at mass-producing other gear that has appeared on their productions... which means stuff like the Buster Sword or "Deus Ex" cybernetics (although... would that make Sarif a division of SHINRA? There's a good question...)
I think kind of the same happens with Weyland-Yutani: everybody knows the kind of (extremely censored) they are, but they have an edge over everybody else when it comes to the actual knowledge (and application thereof) of anything related to Xenomorphs, Yautja, Engineers, Space Jockeys and the myriad rip-offs of them that plague the Multiverse.
It's a pretty niche market (aside from, say, atmosphere processors) but it provides money.
((())))
As for Newtype Ace... ok, I think I have an idea or two for it. Maybe it could be a Prestige Class for Mecha Pilot, for one.
Dunno much about Newtype powers (of the shows and OAVs I saw, the only one I am sure of is part of the Universal Century was "War In The Pocket"), but on those shows most of the time it was like "psychic powers give me eyes on the back of my head", so that's the focus I would be going for in terms of special abilities.
I think there's stuff like psych powers providing juice and guidance for Funnels, or so I've heard from seeing the "Char's Counterattack" TV Tropes page, but again I don't know any true specifics about Newtype powers, so I think my focus will be a general "Ace with some additional power" angle.
And as for Bard... I thought there was a class on Urban Arcana, but I went and checked and nope, there isn't one. Silly me.
I can see two versions: one that is more of a general "rocker" or "impressive artist" in the sense of "can cause flash mobs with his astounding ability to perform art", which I guess can be upgraded to flash riots, and a closer-to-D&D "uses magic music" kind.