since I posted this I've been thinking about what this theoretical signature skill could be, but always ran into the same issue that what I came up with would steal the other party members' niches, which is what siffrin's level up skills in game already do (learning healing to replace mira, learning all craft types to replace odile), and I didn't want that... but I think I have it figured out now.
sif's new signature skill definitely wouldn't be an offensive attack. sif is out of those loops, so they don't have to fight anymore (or not as much). and also one of the issues that arised in the loops was how sif gradually became noticeably stronger than everyone else, to the point that the others became... kinda unnecessary. at higher levels and in act 4, you could pretty much just ignore everyone else and attack with sif only--and in most cases a single (just attack.) is enough to kill all the enemies before anyone else even gets a turn.
post loops sif is still significantly stronger than everyone else, and you can't really ignore that, but at the same time it would be nice to uh, rely on the others more i guess? sif gets to take a backseat and support his family members' damage this time instead of the other way around. to be more equal.
so what i came up with is sif's signature skill being a mirror version of isa's, where sif uses all debuffs on all enemies. knife to meet you and too cleaver by half already have a chance to debuff enemies on hit, so this would be more a natural progression of sif's skill set than him stealing someone else's niche. this also ties into how sif and isa have the same initial skills (single target attack, all targets attack, teamwide buff & giving up their turn). so it's only natural that they get to match their signature skills too ehehe.