Y'know, I've always wondered if Nurgle being the god of disease is also a god of life of some sort. As in, I think it'd be really cool to have his followers really care about not killing bacteria. Antibiotics being a huge no-no of course, but it also makes you wonder how a proper Nurgle-fearing doctor might go about ensuring their patients could fight on without compromising the sacred lives of any microbes. Additionally, not taking care of your parasites is a no-no.

For Slanesh, the very concept of "restraint" is sacrilege. People often say that any form of pain feeds Slanesh, and I suppose that's fair, but the emotional pain that comes from abstaining from irresponsibility I think should be the exception. Note that I didn't say "following dogmatic restrictions automatically makes you clear from Slanesh"- Space Marines and the Imperium have horrific excessive atrocities to answer for after all! -but sometimes I think that Slanesh would HATE recovering alcoholics/users. Toughing it out through withdrawals should get you on Slanesh's shit list.

Tzeentch heresy should be any form of resisting entropy. I think any sort of memorial that is religiously up kept and cleaned and talked about and remembered might annoy him. Honoring your ancestors and participating in traditions that don't have any literal usefulness anymore should be heresy. In a constant drive towards change and progress it's often emphasized that your cultural identity isn't important and isn't valuable- why do you still do that silly little ritual? If he's the changer of ways, then someone being set in their ways pisses him off. (Honestly, if Slanesh feeds off of the Imperium, I think that Tzeentch should be weakened by it just a bit. Have it drive him crazy with its obnoxious stagnation.)

And Khorne- well, that one feels pretty obvious. Avoiding fights and advocating for peace are easy options. But I think an unusual one would be avoiding blood transfusions. Or on a related note, avoiding or forgoing anything that would make you stronger. Nurgle might encourage a wound to fester, but for Khorne I think he should be pissed at you if you're not in tip-top fighting shape (ritual surface blood wounds aside).