This is going to be a tough one. It really depends on whether or not Unity is up to the task. And it would be impossible to introduce the entire language due to international differences (American sign language is radically different from, say, British sign language, even though both countries speak English), regional dialects, accents, and so on.
But if a selection of important key words or phrases could be introduced, you’d add line-of-sight inaudible communication between players. On a raid or in a gunfight, two players taking cover behind two boulders could communicate tactics without the opposing party intercepting. During a reconnaissance mission, a scout could signal back “danger”. Or “lots of resources”. Or “base / camp”. When passing through someone else’s turf, you could sign “friendly” (aka “famous last words”) while sprinting. The possibilities with just a small number of signs would be significant. Fingerspelling would be a huge gain, but might be too complex to introduce. Even 5-10 carefully-selected words or phrases would add an completely new and unique dimension to gameplay. Keeping the vocabulary brief would also allow someone who knows no sign language at all to quickly pick up the signs.
This could be implemented via a selector wheel just like the building plans.
Signing would have some inherent limits. Just like voice chat, signing can be intercepted by someone within sight range, so it’s not automatically a secure means of communication. People could also lie with signs, just as they do with voice chat and text. You might want to consider momentarily putting the weapon away, but a lot of signs can be done and understood one-handed. And there’s the risk the recipient might not see the sign due to distance or obstructions – but these are problems with voice chat as well.
Friendly: http://www.lifeprint.com/asl101/pages-signs/f/friend.htm or http://www.lifeprint.com/asl101/pages-signs/f/friendly.htm
Again, this falls almost entirely on how complex Unity’s hand manipulation can be. The actual signs probably wouldn’t be hard to add as 2-second animations, but Unity may not be up to the job of such precise manipulation. There are some other concerns like it would be harder to read from the side or impossible from behind, but affect real world signing as well, so they’re not an artificial in-game issue.