Assuming legendary was based around two shot sniper jackal kills, I’d make a skull that made them kill you in one.
I’ve suggested before, I’d like to see 2 new skulls added to all of the games:
Invisibility skull: Not the boring envy, but proper invisibility like the camo powerup or equipment in Halo 3, but infinite use/length (maybe on command like the jetpack skull, maybe tapping/holding crouch toggling cloak on/off)
Immortal Ally NPCs
Both of these are mainly because of what it means for showcasing the AI in the games, because Halo has some terrific AI (FEAR-levels), with complex behaviours, not a lot of games have AI senses based on sight and sound, different states like idling, alert, investigating, surprised, charging, retreating, pursuing, flanking, berserking, panicking - Even faction fighting is a bit rare, often devs seem to signal that it’s a pain to code AI that should try and switch targets, prioritise their own targets, given as a reason why some games will not have coop added (because of AI rewriting).
In normal gameplay you can easily miss and take for granted the behaviours that are running under the hood, in the frantic fast paced chaotic battles, opportunities to just observe as AI fight each other, and toy with the AI to make them try and uncover your location and witness how they react, both of those skulls would in my opinion just show how great the underlying AI can be, and be simply fun (predator camo doesn’t really get old and big armies butting heads looks great in Halo battlefields).