I've tooled around with the vanilla Legion a bit and the best I came up with basically amounted to a gimped Onslaught with cheap fighters. The two common criticisms of the ship are true, it's weakened by its split role and it doesn't have enough OP to adequately fulfill either role. Whatever you outfit the Legion to do, another ship can do better.
I haven't used the XIV Legion extensively but a lot of people seem to like it, and I can see the 2x large missile slots turning it into a decent brawler with 2x Cyclones.
Whenever I start thinking about a build for the vanilla Legion I ask myself, "Why wouldn't you just take an Onslaught or an Astral?" If you're building your Legion like a battleship then it's beaten by the Onslaught, if you're focusing on fighters then it's beaten by the Astral.
I know the idea is for the Legion to be a frontline ship but I don't think it should have Burn Drive as its system. I think it should have more top speed, let's say 40, and a ship system that will help it in battle. Maybe Damper Field, or something to buff its fighters? This will also make it more amenable to long range support builds.