>>536454712
>what are the practically mandatory monster girls to have in a game?
Harpies, beastgirls (especially catgirls), fairies, kitsune, centaurs, lamias, slimegirls, and plant girls all feel pretty much ubiquitous in dedicated monstergirl settings.
I also can't think of a single monstergirl porn game that doesn't prominently feature succubi, though they're significantly less common in SFW media for obvious reasons.
>what monster girls would you like to see more of?
Slimegirls are already somewhat common, but in my experience they mostly just seem to be used as inconsequential cannon fodder enemies. I can't think of very many games off the top of my head that feature notable slimegirl characters, which is a shame.
Mermaids also don't feel like they show up as much as you'd expect despite being such an iconic example of a monstergirl, probably because they're generally restricted to water.