Here's a breakdown of the potential characters that was posted a few days earlier on another discussion board...it's organized according to likelihood of each person's participation in the storyline:
The Absolute Indispensibles:
[*]John Sheridan: A no-brainer. President of the Interstellar Alliance, the plight of both the "normals" and "blips" becomes his concern during the conflict, walking the lines between the needs of Earth (pardon pun) and the other races under his leadership.
[*]Lyta Alexander: Another no-brainer. Her uprising long in the planning phases, the Telepath War erupts -- starting causes and party unknown as yet -- upon her return with G'Kar in 2264.
[*]Michael Garibaldi: Connected to Lyta and Alfred Bester in more ways than he'd care to imagine. Put in charge of financing, training, and equipping Lyta's paramilitary force during her absence, he possesses another very personal grudge against Bester due to his telepathic neural block.
[*]G'Kar: Lyta's travelling companion from 2262 until 2264, G'Kar has become an advisor to Sheridan and Delenn as of late 2265 (c.f., Legend of the Rangers); he would undoubtedly inform the IA as to Lyta's intended plans, and have some decent character moments and lovely G'kar Speeches as the plausible voice of conscience to Lyta's troubled id.
[*]Alfred Bester: The third no-brainer. Little needs saying, here.
[*]Lennier: JMS has stated that Lennier meets his fate during the Telepath War, and the early script draft for the Crusade episode "The Path of Sorrows" depicted Lyta Alexander crying out to Lennier during the assault upon the Psi Corps's Mars headquarters. See "Delenn" below for further notes on this one.
[*]Delenn: Her primary reasons for meaningful direct involvement in the Teep War are twofold: first, as Sheridan's "vice-president" (per "A Call to Arms") and head of the Rangers, she would be in a direct position to influence the participation of the Interstellar Alliance's frontline forces in any action against the Psi Corps (again, note that the IA is apparently brought into the war once the alien races and outer human colonies are threatened -- an explicit Alliance jurisdictional affair).
Second -- the setup seen in "Objects at Rest," concerning Lennier's eventual "redemption" in both she and her husband's eyes for his crime. Having Lennier and Delenn and Sheridan cross paths once more against the backdrop of the war -- while Lennier accomplishes some sort of significant act that affects those involved, redeems himself, and which results in his death -- is a vital part of this story.
Potential Vital Supporting Players:
[*]Susan Ivanova: Apart from being captain of one of EarthForce's premier Warlock-class destroyers, and thus likely to be positioned on the front lines of any major military action (and her destroyer having recently been freed of Shadowtech influence that the Corps could exploit), Ivanova holds a very, very personal reason for a probable inclusion by JMS in the screenplay: her family history with the Psi Corps, and the organic character outgrowth of her unleashing her long-held-in-check inner rage, and giving Claudia Christian some possibly significant scenes to play. And again, another tremendous "G'Kar-as-conscience" opportunity.
She and Straczynski have apparently patched up their differences in recent years (Joe speaking glowingly of her on the DVD commentary tracks, etc.), and this forgiveness and rapprochement could translate into a meaty role for her in the flick.
[*]Elizabeth Lochley & Zack Allan: For any motion picture to feature the words "Babylon 5" in its title, audiences will rightly be expecting to see some glimpse of the station itself. As a strategic point for the Alliance circa 2264-65, it might very well serve as a tactical staging area for forces from Minbar and other IA worlds. And its commander and chief of security would be right there in the thick of the preparations to war. (To wit, there is also that long-awaited Lochley/Ivanova Jell-O bitchfight-throwdown that we might end up glimpsing, but I digress...)
[*]Dr. Franklin: As head of Xenobiological Research at EarthDome, Franklin holds an important position close to the President herself, and would undoubtedly be consulted during the war effort thanks to his work both with legit telepaths, and as the former head of the "underground railroad" he helped to put together. This latter characteristic could be the more important of the two, as coordinating between Lyta's resistance forces and the Alliance might prove to be a major turning point in the conflict.
[*]Susanna Luchenko: President of the Earth Alliance, she'd be rightsmackdab in the political (and military) crossfires arising out of Psi Corps's sudden movement to overthrow the government, and the Earth Senate's disassociation with the agency. Lots of fireworks could await, here; Beata Pozniak in fact reprised her role in the ****canned B5 computer simulator, and could be seen here, as well (assuming that all Contractual Actor Issues are sussed out).
Others Who Might Get A Smidgen Of Screentime, But Who Are More Or Less Even Money Right At This Moment:
[*]Vir Cotto: Centauri Ambassador to Babylon 5, it is described a number of times in the "Fall of Centuari Prime" trilogy of novels that Vir was onboard B5 during the Teep War, and as such would have probable interactions with Captain Lochley and others there. If the war comes to Babylon 5 as is expected, his involvement as an "unofficial" advisor to the IA (owing to Centauri Prime's Drakh-induced withdrawal) could give Stephen Furst some good scenes with both the human characters, as well as others (for instance, say...G'Kar).
[*]Londo Mollari: Perhaps the most difficult character to shoehorn into the storyline, it is still possible via one of several methods; see Joe DeMartino's "Drakh arms sale" scenario, for one. Being in contact with Vir aboard the station for some reason is another.
[*]Shiv'kala: See "Londo" for all pertinent reasons.
[*]Tessa Halloran/"Number One": Garibaldi's successor as the IA's Head of Covert Intelligence, she'd be in a primo position to place past members of the Mars Resistance into contact with Sheridan's and Lyta's forces, due to the location of Psi Corps's primary headquarters there.
[*]John Matheson: Maybe, maybe not. But he was certainly on the opposite side of the war for quite some time, up to and during the Psi Corps's probable victory against EarthGov, and the one Crusade character whom could fit into the movie without it feeling totally contrived. Perhaps a brief, passing cameo. We'll see.
[*]Byron Gordon: As Neroon once said, "...After a fashion." According to the third "Psi Corps" novel, before he died, Byron placed a telepathic simulacrum of himself within Bester's subconscious, which starts surfacing around the time of the war as an accusing, Jiminy Cricket-esque, birdie-on-the-shoulder voice of neglected morality to him, much to Bester's woe. Having Robin Atkin Downes doing a few scenes would give some ironic resonance to the whole "Remember Byron" leitmotif, and to the entire first half of Season Five.