Here's the problem I have with Melee weapons:
First, the size of the maps. If you look at games that do have melee weapons and melee capabilities (half life 2's crowbar, the Halo franchise, Unreal Tournament, etc.) They are quite successful, but they lack something that Legions has: Size and speed. Epic flying melee battles, while sounding awesome, would require more timing and setup than makes sense in a game at the pace Legions plays at.
Secondly, trying to get into one of these melee battles is just tactically inadvisable. (Think the sword guy from Indiana Jones, who he just straight up shoots.) If somebody comes rushing in with a sword and shield, perhaps the shield will protect them from the damage from a rocket blast but it won't stop physics. The player's heroic charge will be reduced to a sitting duck, switching to long range weapons as five explosive projectiles scream in his direction. Even if you do get into a massive melee battle, somebody else is going to intervene, which already happens. (Anybody who's played LD with me knows that I'll try to kill their targets, just to streamline the process of making the base safe.) Not only are you suddenly at the very whim of all the ranged weapons once you switch, they're just too far away you to do anything.
Now, to turn right around and make a suggestion. For balance, what might make sense is something like the Shield Gun from UT 2004. You have to choose between having a shield and charging up the weapon to melee, and then doing it, but in this game, its still just too impractical.