With basic Legion, if I do not care much about fighters, then Onslaught can replace it. Onslaught has similar design, without the fighters but better guns and stats. For Legion XIV, the only thing that sort of substitute for it is two Champions. Legion XIV, it can use any large missile it wants effectively, while many ships with a large missile are not friendly to dumb-fire torpedoes (like Apogee or Conquest) or requires Automated Ships to use (for Rampart and Radiant). As for ballistics, it is possible for XIV to use a bunch of small kinetics (in small and medium mounts) and boost them with ePD+IPDAI or Ballistic Rangefinder. Normal Legion cannot do that because its mediums are composite, and probably need to be filled with missiles.
With the way Legion XIV is designed, player can totally ignore fighters (fill bays with 0 OP pods) and build it like a missileboat, and there is no other capital that has mounts like it has. If player wants to ignore fighters on standard Legion, just get the Onslaught instead.
I am not sure upcoming Pegasus can replace Legion XIV since it costs 50 DP instead of 40, and the mounts usable for dumb-fire torpedoes are widely spaced apart like the Eagle's small missiles.