So if the Railgun returns would you like to see it buffed or changed in any way?
I'd say slightly increase its ammo capacity, double or even triple knockback taken from it (but not damage), raise its blast radius by about 150%, but
distribute the current blast radius damage evenly throughout that. Keeping the actual damage proportional over a larger blast radius would start to cause overlap into high-yield explosives territory like the Concussion Rifle, Brute Shot or Rocket Launcher. If you have reduced damage, but over a larger area, that combined with high knockback allows it to be a persistent harassment weapon that can take out infantry and
dispatch vehicles, but not outright
destroy then. With all the movement emphasis in Halo 5, this also allows you kill momentum in certain situations, for example hitting a wall right next to a player making a jump for a power weapon only to knock them away from it, rail-jumping like you would a rocket jump, blasting a ceiling to send a player exponentially downward, etc.
Isn't that essentially just Thruster? I don't like the idea of getting "dazed" if I miss a melee but the whole side stepping to get out of danger concept sounds like Thruster.
I had thought up a rush-counter thing before Thruster was ever really a thing, but the key difference in this case that it's more of a personal melee-style utility rather than an "outrun the fireball" thing the way Thruster is. When I say "dazed," I don't mean stunned - that would slow down combat - it just keeps you locked into Sprint for an extra second or two, meaning it uses your own tactic of advancing forward against you. It could also serve an important role in countering the weird sprint-punch thing we've been seeing as a Spartan Ability, which would send enemies even further if properly countered.