Fan Theories volume four

Fan theory four:

Science fiction television programmes featuring humanoid aliens are all set in the same universe as the Culture. The difference is that these programmes portray aliens as humans would imagine them to look like. The events portrayed are after the Culture makes contact with Earth but Humans can't take there being other humans "out there" that look just like them so, in their minds collectively, aliens look a lot different.

This theory also allows for the Drakh and Vorlons to be offshoots of the Culture, or that Babylon 5 is actually anti-Culture propaganda. This reflects the policies of the two races and the mobile nature of their civilisations.

Fan theories/ Marcus Cole

Fan theory number three/four:
(theory one being http://sciencefictionshownt.blogspot.co.uk/2017/04/i-wonder-if-it-is-possible-to-explain.html)

Marcus Cole (Babylon 5) is either:
-His brother
or
-A Deep Cover agent working for Earth Force

Theory three allows for Marcus' apparently short training regime with the Rangers by assuming that Marcus didn't join the Rangers after the death of his brother. Instead, this theory assumes that he was among the first humans to be recruited by Sinclair. His psyche invented the dead brother when Marcus heard about the destruction of his home colony during his ranger training.
This theory allows for a longer, more realistic period for the character to become a Ranger.

Theory four incorporates the above somewhat, but posits that Marcus learned the Minbari language before joining the Rangers. It is possible that he visited Minbar some time after the war as a representative of his family's mining interests and learned the language there, but his online biography strongly implies he learned Minbari when he was drafted into Earth Force in the last year of the Minbari war. 
Marcus could secretly have continued working for Earth Force in an intelligence gathering capacity and this led him to join the Rangers in order to spy on Amassador Sinclair or the Minbari military.

Appendix: It is conceivable that the jump gates in Babylon 5 are actually time portals. When a ship enters "hyperspace" it actually travels back to the beginning of the universe, or at least within the first hundred million years or so before the first stars and galaxies formed. When the ship returns to "normal space" it has covered a considerable distance simply because the universe has expanded in the intervening time period. 

Appendix two: What if Delenn was meant to become Kosh? Evidence for this can be seen in the dress of the Minbari assassin in "The Gathering", Kosh's own apparel as viewed by Lyta Alexander when she scans "him" and Delenn's sleeve in the episode "Babylon Squared", when future Delenn and an aged Sinclair observe present Garibaldi and present Sinclair leaving Babylon 4. This also explains why Dr Kyle recognises Kosh when he opens Kosh's encounter suit and why Kosh says everyone would recognise him. Delenn could become Kosh in order to retroactively create the Vorlon race, assuming Kosh ages backward, in order to prepare for the Shadows' return.

Fan Theories

I wonder if it is possible to "explain" various science fiction franchises by placing them in the same fiction universe as other franchises?

Childhood's End provides the intriguing scenario by which the remaining Humans created Battlestar-style cylons. These cylon characters ended up in various franchises, as evidenced by the cylon ability to "project" a reality they prefer over what actually exists.

In this case the whole Battlestar Galactica saga could be Gaius Baltar "projecting" the entire story. Or any time a character in science fiction hallucinates i.e. when Doctor Stephen Franklin (Babylon 5) meets "himself", Riker and Picard's interactions with Q (Star Trek: TNG) and Batman's many encounters with the character "Batmite".