One thought - some things that are now seen as slurs were never meant to be, and some things that were meant to be are no longer seen that way. There is no reason not to continue this trend.
For example, the term "Yankee Doodle Dandy" was mean to be a slightly vulgar insult, suggesting that the revolutionaries were simply amusing themselves by playing at being an army, the "Dandy" part mocking their lack of formal uniforms. The British never expected the rebels to embrace the term and make it a children's song...
Another example: I've seen the term "slag" crop up in fiction a lot in the last few years. Sometimes it refers to what it also does in metallurgy - a waste byproduct, so calling someone a "slag" would mean they are a useless person. But I've also seen it used to refer to someone who is very promiscuous and would probably not get any attention if they weren't.
If you're dealing with a typical fantasy world, you could also use terms to reference outside forces (infernal, heavenly, celestial, abyssal) as swear words or at least derogatory terms, either used ironically or to suggest someone has ties to one side or another.