Just normal signs would work, Far Cry 2 had different colored arrows for different teams and had extra signs which could be used such as CTD / Uprising or Ammo so you could place an arrow pointing to the direction the diamond was in.
What some might not remember is that some of the signs in far Cry 2 only worked in their actual Multiplayer Modes so as an example an Uprising or Capture The Diamond sign would only be visible for that mode.
Some people exploited this, I remember SupaScoopa_ on PS3 making some ramps out of signs so when you played in the specific mode the ramps could be used bringing another entry / exit area into play - very clever.
If we simply had Up / Down / Right / Left arrows say on a board which had a good view-able distance so they didn't pop in and out of view when really close like some of the items do - then I see these as been very useful.
The Gold Bars get used a lot in Far Cry 4 for making custom letters, if they give us more neat items like that with good draw distance I am sure those with patience will make their own signs.
As for lettering if we had Scripting and a way to input Text like in Gunscape when you walk to a set point you could have a message or text show up on screen and set it to once only or every time so each player only sees it once or it will show each time they cross that area. Obviously it would have a language filter built in.