I'm back with some basic ideas on what the primary races of B5 might look like, technologically, in a 'hybrid' universe, and how they might fit into the grander scheme of things as far as the future of the Federation and whatnot.
I came across a suggestion online that when JMS originally took his B5 idea to the folks at CBS, he did so with it being a spin-off of ST, and with the Centauri being the 'smooth-head' Klingons from the original Star Trek. I actually like the idea behind that idea in principle, so in my little 'hybrid' universe, the Centauri are a fairly aggressive and prideful people who are continuously trying to expand their territory in order to get back what they once held but lost due to an inability to keep all of the territory they'd laid claim to, which is an attitude that will inevitably bring them into conflict with other people, particularly the Federation. They'll have an encounter - and skirmish - or two with the Enterprise and her crew, but won't really become a major player in universal events until much, much later. I also see their current territory being fairly close to Klingon space, and them having an alliance of sorts with the Klingons because of similarities in their desires for conquest and expansion.
When I think about the sort of role that the Narn might plaly in a 'hybrid' universe that melds and merges both B5 and ST, I think of them as sort of a cross, culturally, between the Cardassians and the Ferengi, with a bit of the Klingon superiority complex factor tossed in as well, which is an angle that I think could fit very well into a tale of the formation of the United Federation of Planets, particularly in terms of them being weapons dealers. Since they're a rebuilding society, their space-faring tech is fairly primative, but they're still able to travel sufficiently enough to serve as arms dealers and make contacts with people. I can also see them working with the Orions, Ferengi, and other races such as the Nausicans and basically making up the 'criminal contingent' of the universe's various alien powers.
I sort of see the Minbari as being parallel to the Vulcans, albeit with a bit more mysticism to them and their culture. I see them as being a fifth pillar in the founding of the Federation, but don't see them really playing a major role in universal affairs until after the events of the Romulan War. Technologically, they're also the most advanced of the races, which is another reason for them not to have anything to do with major universal events until after the Romulan War. Alternatively, they could end up being the first race that Earth encountered - in place of the Vulcans (who would then be the second race that Earth encountered), although it might be tricky to figure out how to prevent Earth from becoming sufficiently more techologically advanced than I'd like them to be if the Minbari were the first race they encountered.
Someone above suggested possibly substituting the Dilgar for the Romulans, but I actually like the idea of the Dilgar and Romulans being allies and 'sister powers', with their home territories bordering each other, and the Dilgar providing the Romulans with resources that they might not otherwise be able to obtain. Technologically, I can see them being very similar to the Romulans, which puts them on par with the Centauri and Klingons as well.
I also nailed down where I want to go, technologically, with Earth. Not counting the Enterprise, all of Earth's ships have basic FTL tech (although nothing like the Warp 5 engine), plasma-based weapons tech, and the hull polarization defense technology that was seen on the NX-01 in Star Trek Enterprise, whereas the Enterprise NX-01 itself has the aforementioned Warp 5 engine and the first prototypes of phaser and shielding technology.
I've also decided to limit the variety of design in Earth's ship technology, with a lot of their ships either falling into the Daedalus, DIY, or 'Ring Ship Enterprise' design categories from ST or the Hyperion-class design category from B5. This doesn't include the Enterprise NX-01, which is more of a hybrid between something like the Intrepid class from ST and the Warlock class from B5.
In terms of culture, I'd like the pre-Federation Earth society to be somewhat of a hybrid between the ST utopia and B5's more realistic take on things societally, since I think it gives you somewhere to go, from a societal and cultural standpoint, narratively.