In the last week I sat down and blew through all 5 seasons of the series.
!!! Damn!
I think O'Hare (Sinclair) also left because he was afraid of being typecast. By all accounts, the parting was amicable.
The idea of a Sheridan came up during season 1. JMS was thinking of making him the guy who's out there fighting the Shadows while Sinclair is taking care of stuff on the station, dealing with Earth and his Valen-ness and the Rangers. When O'Hare wanted to go, he just sent Sinclair to Minbar where he could do all the Rangers and Valen stuff off-screen, come back later with B4 and get sent off to his proper heroic conclusion, while Sheridan becomes the Shadow fighter guy.
Of course this was all decided after shooting Babylon Squared (season one ep when B4 appears) and so one would assume that the ghostly traveler in the blue suit that Sinclair touches and flies back from was planned to be Sinclair. By the time War Without End came (season 3 two-parter concluding the B4 arc) they made it Delenn and had to concoct the switching suit/stabilizer plot elements to accomodate this change, as well as the "three Ones in one" speech by Zathras.
(among other things, both had mates that got themselves in dangerous places--ie, it could have been catherine who went to zahadum and got killed)
Yes- had Sinclair stayed on, Sakai would have gone to Z'ha Dum, been captured by the Shadows and sent back to lure Sinclair.
Have you seen the pilot, "The Gathering?" It takes place a year before season and was also filmed a year before seson 1 was filmed. A lot of it is sketchy in terms of quality but required viewing for a fan. It stars a different doctor and second in command (the actors were unavailable for season 1 by the time they got around to making it, hence Ivanova and Franklin). It also had Lyta, not Talia, and shows the incident of her connecting with Kosh that made her get involved with Vorlons in the first place.
It also has a different girlfriend for Sinclair who has a similar job to Sakai, which suggests that no matter what, whoever's running the station needed to have the girlfriend become a Shadow zombie. The only difference with Anna Sheridan is that they made it someone from his past, allowing Sheridan and Delenn's romance to develop slowly. Some fans (including myself) speculate that Sinclair and Delenn would have gotten involved as well, but JMS denies it.
And while I'm recommending B5 movies, check out In the Beginning if you haven't. Broadcast after season 4, it tells the story of the Earth-Minbari war and chronologically plays out the events of this time period that a one pieces together over 4 seasons while watching for the first time. Other films, like Thirdspace and River of Souls, are stand-alone stories with varying degrees of quality, while A Call To Arms was to act as a lead into the defunct Crusade spin-off series. All the films are available in a box set, while The Gathering and In the Beginning can be found on one double-sided DVD for real cheap.