Just as the title suggests: Are there any particular or unique perks you're wanting to see either brought back from a previous Fallout game, or a new one created that would further a game mechanic in FO4 specifically?
I'm personally interested in seeing improvements to stealth-based game play; with respect to that niche I'd say "Sandman" Perk comes to mind.
I'd also love to see you be able to kill silently from behind with a knife or other suitable melee weapon. Hiding bodies would be good as well, with the added improvement of having there be a reason to hide bodies e.g. a better AI/ alarm system. . .