Could you suggest any options involving the slug? I can't think of any that I've seen before.
Not really a fan of slugs or related creatures because they are a bit "niche" taste I guess + form is more limited.
The most common h-tropes I've seen are:
- birth/unbirth - the most often I've seen, ranging from one slug entering in/out (Iris Action / Iris Chronicle) to a full blown breeding seedpod (Sinisistar?).
- missionary
- soft and hard vore
- nipples/vagina sucking (parasitism)
- brainwashing/braineating - mostly doujins delve into that territory but Sinisistar has some taste of that.
Sinisistar 1 and 2 had quite a sizable number of slug-like enemies that would follow this list + some brainwashing/braineating activities. I'd recommend it if you are into this kind of enemies (at least one level is purely dedicated for slugs alone, with mega slugs).
Iris Action and
Chronicle have some slug/jellyfish enemies with really good animations.
Guilty Hell 1 and 2 has one or two enemies that would fit the bill though it's rather meh with these enemies.
Agnietta ~The holy healer & the cursed dungeon~ and
La Vitalis : Immortal Loss (both by B-flat) are somewhat oriented around slugs/tentacles/facehuggers and such in forms of different parasites, though I dislike the gameplay.
MonstersAbyss could be interesting with the wide choice of enemies (including one slug-like) and the animations but the way development demos are presented in infuriating for me so I wait till it is finally released.