In the rulebook, the baseline is not -- contrary to what most people think -- the line between two bases. Rather, it's a straight line between wherever the runner is and the base he's going for when a tag is attempted. As the MLB rulebook puts it:
A runner is out when: (1) He runs more than three feet away from his base path to avoid being tagged ... A runner's base path is established when the tag attempt occurs and is a straight line from the runner to the base he is attempting to reach safely.
If no defender is attempting to tag the runner, there is no baseline, and the runner can go anywhere he wants. He can walk into right field if he wants.