Slimes being able to seamlessly enter your anus without stretching and intensely, completely stimulate only your prostate is incredibly hot. Also, them being able to form into a facsimile of multiple women, or a formless gigantic blob/lake of sexual body parts is great.
There's also the idea that by pooling their slime inside your seminal vesicles, and then forcing it to gush out, they could cause a sensation of constant ejaculation regardless of refractory periods. I've only seen it in a scarce few images though, wish it was more common.
Way more slimes should be a translucent white instead of bright primary colors, to more accurately reflect their diet; With herbivorous/carnivourous being green/red, and the ones you want to find being pale white