GTA Forums' user funmw2 has discovered new files in Grand Theft Auto 5's executable which seems to confirm upcoming single player DLC will feature assassination missions and new features which may include new phone apps or special abilities.
While assassination missions will form part of the DLC, there's no reason to believe that Rockstar will have such a limited focus. Rockstar has said:
"For those ready to jump back into the story of Grand Theft Auto V, we have big plans for substantial additions in 2014 continuing Michael, Franklin and Trevor's action, mayhem and unexpected adventures in Southern San Andreas."
Although there may be a new special ability this does not, however, mean that there will necessarily be a new character.
It also seems that a shovel and wrench may be added as weapons in the forthcoming DLC. funmw2 leaked the game's content creator a few weeks ago so there may be some truth to these rumours.
Hopefully we'll know more soon, what do you want from GTA5's single player DLC?