Stealth should certainly be in there, but I hope they can come up with some new ideas because games have been rehashing the same stealth mechanics for years.
I would really really really like to see stealth implemented
exclusively through use of active camouflage or something similar. Ditch the crouch to hide mechanic, having that plus camo is redundant, and the crouch thing is overdone, slow, boring and uses up button on console. Flip on your camo implant and your hidden, like ghost in the shell.
Camo implants would need to be balanced, having a time limit or power consumption like deus ex, and you shouldn't be 100% invisible, just like the crouch mechanic it would replace. Line of sight, bumping shit, and sound/footsteps would also play a role, but nothing's stoping you from walking freely and sprinting away or at someone's throat if needed. Upgrades could maybe dampen footsteps, and make camouflage more efficient, and maybe a way to hack sensory implants.
The whole system should be realistic, weapons that go bang alert enemies, silent ones don't, unless your really close. And I don't want a HUD hidden/seen indicator, I should look around and any npc that sees me and gives a shit will be staring at me, and certain npcs won't care, crime is too common. This excludes big stuff like murder were you should be alone, whether your sniping on an abandoned roof top, or down in an alley. All intuitive stuff that would be common sense in real life. Also guards should react more sensibly, if they find someone dead they become uber paranoid, and when/if they stop looking at you, they still are alert and suspicious. The Arkham games do this well with heart rates and npcs firing desperately into the shadows.
And lastly as a personal request, you should be able to climb across ceilings, like Tenchu Z, to sneak past enemies or pounce kill them.