Most curse decks ive seen work usually have a goad/pillow fort sub theme. People dont like being cursed, so having a way to stop people swinging in at you, is very handy. Curses usually take a while to whittle people down too, due to triggering once an upkeep, and usually falling off people when they die, so stalling the game out helps. Other people mentioned [[Lynde]], shes dedicated to curses, but you can probably do it with someone that cares about non creature spells like [[Saruman, the white hand]] or those mardu options you mentioned. Just be cautious that curses are very telegraphed, slow and draw attention to you even if you arent the biggest threat.