Things I'd like to see in a "Companion"
...price guides - not only things (those are obvious - all kinds of things), but also services, from an taxi ride to an haircut, from an hour of psychotherapy to repairing your starship's front bumper, from an telepath's fee to an night with an exotic dancer - what does it cost where...
...job tables - how much does it pay, how much time does it take, what do you need to do it, what do you need to get it, etc...
...everyday B5-time operations - how have things changed from what we all know, from postal service to spaceship check-ins, from hotel rooms to home entertainment, from emergency procedures to phonecalls.
...legal stuff - what is allowed and what not; and where (no weapons on B5, but what about elsewhere in the EA? and which weapons - how small a knife is allowed as tool, and when does it catch the eye of Mr.Garibaldi) where Exactly does the law draw the line (as I'm sure eventually PC's will get there), and what fines/punishments do PC's have to dodge, where and what for they can/have to get licenses and permits, etc...
...illegal stuff - how's the crime situation, who can be bribed with how much, what's currently hot on the black market, where and to what conditions can one get rid of stolen goods, which organizations do you have to watch out for, etc.
...travel guides - including short "PC knowledge" descriptions of lots of places and races, currency conversions, and stuff the travel agent is by law required to tell you...
...travel hazards - from space terrain over wierd infections (and what can be done against them) to strange new foods that have surprising side effects no docter warned you about...
...rumours - what are people saying about the Drazi in the corridors of B5, which scandal is bound to hit the news any day now and what has been heard about the strange things seen in sector 857... (not everyone will have enough knowledge of B5 to improvise something that sounds likely...)
...bargains - from the B5 Zocalo (what you can find - "hey, look this seems to be some Centauri por... GAH!", and at what prices, how you have to deal without insulting certain races, etc.) to shady deals with Thrakallan smugglers...
...lies - a small section about politics in and around B5; with a little info on all the ambassadors and governments.
Things I actually wouldn't like to see there:
Everything that deserves to or is already planned on getting it's own book.
...Ranger stuff, planets & maps, ship stats and deck plans, ship combat & construction rules, detailed NPC descriptions, detailed race descriptions, alternate history stuff, time period stuff (after all, the seasons will be covered in their books already, and I'd presume Mongoose will do more books liek them covering the movies and Crusade...)
Everything that cen be gained from other d20 products (I'm sure somewhere people can dig up more then enough d20 stuff about everything from our modern age, including martial arts styles... so that'd leave only the alien martial arts...)
Soo, that's what first came to my mind.