The smg and the plasma rifle. These 2 weapons have a reputation of being very popular amongst the community. You could say that adding them in the next halo game would be redundant, but that doesn’t matter. They only need to come back under one pretense: fan service.
Sure it would make the weapon sandbox less unique, but what if in the next game, the sandbox is very unique? What 343 strikes a creative gold mine and manages to make some crazy weapons? At that point them being in the game doesn’t hurt anybody, unless its imbalanced, but balancing two guns shouldn’t be too hard. I mean really, the smg would fill the role of the suppressor, while the latter would be replaced by a more unique promethean weapon. The plasma rifle would replace the plasma pistol in loadouts as a secondary, then the PP can be put on maps. Everybody wins here, dual wielding doesn’t have to be in the game to make them effective.