Since Hellboy is literally a half-demon, an R-rated Hellboy film would allow the themes in the original Hellboy to be more realistically depicted. Hellboy is a character caught between an infernal world and the terrestrial world. In the new film he seems less certain about whether he is good or evil which makes sense if tradition is followed regarding demons. They they are usually at least forces of chaos if not evil.
An R-rating will also allow for a more dramatic depiction of the inhabitants of an infernal realm such as the Medieval depiction of Hell. On the other hand, an R-rating might cause some fans to think less of the film since it will inevitably involve darker and more morally grey elements. More important is how well the script is written since typically the main problem with many reboots and sequels involves whether story can stay true to the characters and the original themes that drove the earlier story.