I’m bored so I’m posing this two-part question. Are there genres of literature that are under-represented in IF and if so, what are they? 
Nonfiction, for obvious reasons.
When I go to Amazon and click on Literature -> Genre Fiction, I see this list:
Action & Adventure (180,408)
Biographical (10,488)
Coming of Age (21,640)
Family Saga (21,276)
Gay & Lesbian (34,502)
Historical (119,271)
Holidays (1,823)
Horror (105,800)
Mashups (313)
Medical (4,363)
Metaphysical (7,780)
Movie Tie-Ins (9,365)
Political (12,656)
Religious & Inspirational (56,963)
Satire (6,874)
Sea Adventures (5,804)
Sports (4,104)
Urban Life (9,805)
War (24,161)
Westerns (35,739)
Leaping out at me: biographies, holidays, medical, political, religious, sea adventures, and sports.
We have this conversation about once a year, it seems. Other genres that have been brought up in earlier years are Superhero and Romance, although I think Twine has helped raise the number of the latter.
Really depends on what you mean by ‘under-represented’, and what genres we should be comparing it to. (TV genres? Novel genres? Videogame genres? Morality plays?) And this sort of discussion easily turns into ‘which genres do I want to see more of,’ in which case any genre can be considered under-represented just because there’s not enough of it for your taste.
The stuff that is well-represented is, broadly, Genres That Geeks Like: science fiction, superhero, fantasy, horror, mystery, certain flavours of comedy. Within those, there are things that are a Big Deal in other media but aren’t super-common in IF, because of particular difficulties of scale or character-simulation that they pose: we don’t have a lot of large-scale space opera or epic Tolkienian fantasy because, well, making really long-arc IF is difficult.
Then there’s stuff that’s to do with the demographics of the community. We don’t have a great deal of postcolonial literature because we don’t have a lot of members from postcolonial nations. We don’t have a whole lot of traditionally-female genres (romance, urban fantasy) because our community’s predominantly male and (gender discussion goes here) (and also, to at least some extent, because NPCs are difficult.) We don’t have a whole lot of hard-nosed military sf or Tom Clancy-esque political-action-thriller stuff, partly because Violence And Intrigue Are Difficult, partly because the community is generally a good deal less right-wing than the primary audience for that.
There’s not a huge amount of straight historical fiction. I think the fantasy just kind of gloms it up.
Then there’s the fact that we’re not really big enough to split into different genre-oriented communities, and that some genres work much better in their own space. I suspect that this is at least part of why there isn’t a huge fanfic tradition: fanfic works best within a community of fans. (Twine confessionals and AIF, in quite different ways, are genres that work better in communities oriented around that particular genre.) Similarly, religious fiction.
I think there’s lots of under-representation on things that early IF couldn’t do well. The limitation of the 80s cascades to today. Things like real-time, conversations, spatial relations, groups of people, fire. As a result there are fewer text adventures that map to action movies, disaster movies, courtroom dramas, true crime-style works and whatever spatial relations would map to. BANGING, I guess.
You rolling around ideas in your head for a new game, MTW?
I had nearly completed my follow-up to my 3rd place IntroComp entry but, after having almost finished it, my Inform glitched out on me and I lost it to time and space, sending me into a depression and writer’s block. So, I did ask that in hopes of stirring something creatively in me, but this block/depression/whatever is pretty bad lately. TEH DESPARE!!!
There’s a sort of periodic table of storytelling that’s been linked to a couple of times in the recent past, maybe that could help you?
In gruff piratey voice: Bah, yer gots three days ter go till that thar deadline! THREE DAYS! Rewrite it from scratch, ya lily-livered varmint 'fore ye gets me boot ups yer ass!