To help mitigate ambushing solo players, law-abiding citizens can call for help. A small posse of NPC gunmen show up and all “good” players on the map get a notification that another player needs help.
Thing is, there’s no reason to do PVP in free roam besides being an asshole (in most cases). So either you find ways to prohibit it entirely, or you develop a system that feeds back into the Wild West setting.
And outlaws could be visible on the map just like hostile players are currently displayed.