Now, I know that many people like playing dungeons and dragons, and other tabletop RPGs, simply to beat up monsters, so to speak. But for those who want to explore interesting human themes with narratives, this idea, might be good for you. Out of all topics you could do, like the nature of evil or terrorism, racism stands out as an easy, deep and interesting theme. It can be easily implemented. One way is having a very good NPC, self sacrificing, kind and generally good, be racist, against whatever you want. Be it gnomes, elves or orcs. This would make for an interesting enemy, and moral quandary. This can also lead to a monster campaign as mentioned in the previous post
The bullywugs are coming! (Alan Hunter, from the AD&D Fiend Folio, TSR, 1981; originally created by Luke Gygax and Gary Gygax.) Bullywugs appeared in every edition since 1st ed AD&D, showed up in the D&D cartoon, and their 5th ed stats are available in the free Hoard of the Dragon Queen Online Supplement.