Yes if it was like the plasma sword in the Halo games and if there were some missions heavy with an enemy like the Flood, Thorian Creepers or Husks. And if it was a 'one hit kill' weapon like the Halo plasma sword.
But that's just wishful thinking.
A physical bayonet or combat knife (instead of an Omni-blade) would be pretty good and better/more practical than a sword.
I guess they'll just stick with the Omni blade though, and if they do I just hope they change some of the heavy melee animations for some of the classes. I mean the back-hand' heavy melee looks like crap, imo, it looks like your playing tennis not trying to kill something.
All that said, and even though I like playing the Shadow in the MP and one hit killing Phantoms on Gold with a sword, in the end I'd prefer better/newer guns.