I'm sure you've heard this plenty but possibly add in a assassination from behind mechanic? Either if they're at half health or even just one shot if you get behind them. Although portals might make the one shot assassination to op.

Also maybe a camo grind for guns? You don't need gold to be the final camo like every other game. just something for variety for those who can't spend much. Something to keep us going after we get all attachments. Love you guys best game ever your doing f****** great!!! <3 FAVORITE GAME OF ALL TIME (besides maybe gears of war)