Games like Aceonline: DuelX

If you finished Aceonline: DuelX, the closest matches are Raptor: Call of the Shadows, Earth Defense Force 4.1: Wing Diver the Shooter, Ghost in the Shell: Stand Alone Complex - First Assault Online, SRD: Super Real Darwin and Space Tripper. All 9 share its shooter genre and are ranked by how closely their themes and modes overlap.