Also, As for time travel think of this.
Main timeline, you want to invent time trael to kill hitler so you make machine and plan to go back and do it. You activate machine and go back to whenever and kill him. Now here's teh rub... You actually succeed. However the INSTANT you accomplish a major change in the time stream everything changes except you (you are imune BUT only you cause you are inacting the change). Now unless you can travel forard youself with a portable device you are screwed cause the reason for inventing time travel has been eliminated, in fact because of teh changes you made you probably will NOT be born (as well as millions of other people will never have existed). Because families stay togeter, different people meet and have kids, ect... for fifty or so years... Best senario you travel to the future and no one knows you unless they are still alive and you interacted with them back in the 20's or 30's or whenever you went back to... Would you really want to live in a world with no family? Possibly... Here's another example, instead of changing the MAIN timeline (with hitler) this is what happened. You go back in time kill hitler and return to future. Nothing changed??? Wrong, in order for you to return to your future nothing can change, BUT you did kill him so an Alternate timeline was created the branches off the main timeline at the point of the killing. That alternate timeline will be radicly different as I described above. The paradox is that you can't change just one event without affecting EVERYTING after that point UNLESS you have power on the scale of a "Q" type entity... SO this really ruins the idea of the GTVA having that kind of power.
Now something more reasonable.... GTVA invents a form of time travel (maybe a small window of travel , either can only go back 7 days, ect.. or they are auto-returned in a few hours/minutes, can only visit any particular time again just ONCE, ect...) before a jump they do research for minmal changes (don't want to make the GTVA dissapear, or any major victories go poof!) If they need another ship they loo in the files for a missing or similiar event, maybe even a tragic accident. they go back and tow ship back to future) history still records that ship and crew gone, butknow have second chance in future with new destiny and further service to GTVA, that is an AWESOME power to contend with. They can't go back and redo capella because all the years/decades? since capella would be changed. See what I mean, think small and use history to make huge gains without major changes in timeline... Now if next week the shivans destroy a world and the GTVA invented this tech a year ago, they can record the incident, go back and show the proof to command, and evacuate the world OR plan a battle. The amount of time for an immediate jump after the point of time travel invention is critical cause the people involved in thinking of it, designing, and building it actually did create it and know of its operation (ie seven days, code word conundrum means a timeline change has happened, agent will brief staff as to what will happen within 7 day window) ect...
Even with these limitations it is one POWERFUL ability!!!
(Timeship premonition from ST Armada)...
to digress further I love teh recording on B5 when they see teh whitestar kill a shadow ship near B4 so they know they HAVE to go to the past to make that recording true (history MUST unfold, haha that's a paradox, they have to time travel to keep history intact! hehehe...)... The best part was Sinclair getting a letter in his own handwritting from 1000 years ago... (dude it's me, or rather you... YOU will become Valen, you must become the greatest Minbari ever and defeat the shadows or the next 1000 years will change LOL!!!) I can just imagine giving that order to the priests (this box must NOT be opened for 1000 years and only be given to one called sinclair, don't worry he WILL be there at that time, trust me (ha ha!)... My rant is done...