Well, TBH, Earthforce really has only been involved with what, the Dilgar War, Earth-Minbari War, Earth Civil War, and the Drakh War.
I'm not sure if they were actively engaged in the ISA-Centauri War asides from that one Centauri cruiser that tried to blow the Epsilon 3 jumpgate, and as far as I know, Earth was left alone by the Shadows and Vorlons during thier conflict, and sat on the sidelines with everyone else while the Narn and Centauri blasted themselves.
If doing a proper 'military' campaign, it's going to have to be during a war since people can only take so much of shooting Raiders or the occasional rogue space ship. With the Dilgar War, you're hampering yourself because there are like TWO Dilgar ships, a cruiser and a fighter. EM War has been done, and Earth Civil War is a little boring (for me anyways). With the Drakh it's hard, because they launched a massive attack, and then just kind of disappeared an were only seen for two episodes of Crusade.
If you play as the Centauri, you could probably make just about anyone your enemy save the Shadows, and Minbari (I would consider that judging by the wreckage of ships in orbit of Centauri Prime during Into the Fire, they tried to fend of the Vorlon planet killer).