Okay, before anyone gets their armor in a bunch - the Halo 5 Multiplayer Teaser had a Battle Rifle without a scope. It could be a different gun but it made me start thinking about scopes and attachments for guns, along with different firing modes.
Pressing up or down on the D-pad could allow you to switch the mode; with the faction of the gun changing what happens.
For human weapons, this would add a scope or attachment.
Like adding a 2x scope and reducing the reticle size/accuracy to an Assault Rifle or SMG, taking the scope off of a BR or DMR and changing the fire mode, taking the scope off of a sniper rifle and increasing the reticle size and reducing the accuracy (making it better for snapshots and CQC), and maybe even making a Rocket Launcher fire both missiles in a corkscrew pattern and increasing their speed.
For Covenant weapons, this would change the firing mode, just not drastically. Maybe making the Needler fire in 4 round shotgun bursts instead of full-auto or maybe firing one massive needle, changing a Storm Rifle from slow and powerful shots to full auto spray, or making a Fuel Rod Gun fire a massive green laser at the cost of all 5 rounds (like a Hunter).
And Forerunner weapons would have their uses and firing modes completely change. For a Light Rifle it could make the burst or single fire modes stay permanent without scopes, a Scattershot could go into a kind of slug-fire mode where its range increases and it fires all of its shots in a tight burst, and the Incineration Cannon could lock onto up to 4 targets and fire small homing rounds.
This is all just speculation and strange ideas, but I think it’d be kind of cool if it was true. It’d also probably wreak absolute havoc.