Now that fireplaces are implemented, you can find players' camps during nighttime or use it as a signaling instrument in the dark.

No means of visual signaling currently exists in daytime, except for weapons discharges and other noise, i.e. solely auditory. I guess smoke grenades might be planned but would be a rare find, also these only generate "cold smoke" that does not form a vertical pillar.

Signaling fires that are constructed by adding grass/leaves to fireplaces would generate "hot smoke" that forms a visible smoke pillar that extends over the tree crowns.

It's widely known that clear long distance signaling, such as gunfire, has massive potential to attract nosy players who only get a general search direction based on where they heard the gunfire from. Smoke pillars would provide an exact location of activity that most certainly would be thoroughly investigated by curious players of all types.

Teamplay, lonewolfs and base-building would greatly benefit from this mechanic, in ALL kinds of ways, e.g. detection, calls for help, beacons, navigational aid, bandit traps, diversions, invitations, scare tactics, trails, bait, calling cards etc. etc. etc.

I recommend the smoke duration to be roughly 10-30 minutes. What's important is that it should enable as many uses as possible, like the ones previously suggested. A variable amount of grass/fuel might be considerable. Type of smoke might also be selectable, e.g. thick black or light gray.