Most underused genres?

"godlike" Shmups/STGs

Wargames, as in histwar, ageod, hps simulation and matrix studios games, as in 99% of people will just scream "they forgot to add graphics" and "i have a headache". Its not really an underused genre. Nore a niche one with a particular kind of players.

Long Live "War In The Pacific Admiral Edition".

fps like call of duty (?)

High speed mobile suit games. (Gundam Extreme vs, Zone of the enders.)

goat simulators, we need more of those.

ohh and sheep simulators, cow simulators, simulator simulators...

Naval tactical sims, like Harpoon, Jane's Fleet Command or Fighting Steel.
There is Naval War: Arctic Circle, but constant crashes combined with lack of in-mission save ruined it.

there's not many life simulator apart from sims and some browser games there's none

Age of Empires 1 and the expansion

need more RPG's (like elder scrolls or fable) with co-op

Tower defense, love them and I need more!

Turn based RPG games.

Third person shooters, especially with good coop mechanics.

More vampire action/rpg games ( like bloodlines). And no, DARK doesn't count. WoD specifically but sadly that isn't happening.
More RPGs like BG/NWN/PS:T. I know pillars of eternity and torment are coming out, but until I play them I can't really judge them. DA:O could be counted in that category, but DA:2 wasn't.

I really enjoy games where you can train AI to do something. Black and White creatures, some old RTS allowed you to assign AI commanders to build you bases/attack/defend/gather resources for you. Rainbow Six series where you could simply enjoy an observer role as the AI relied on your specified route to complete the entire mission.

Basically I would love more games where you could train smarter AI to do your bidding in completing the game. An RTS where the troops had actual combat smarts and you could simply say "Attack in this direction" or "Assign this flank under AI" would be my dream. Like I build and hone the army composition and watch the AI do the actual fighting. I love the feeling of a good prep job in a game and do enjoy looking at the combat more than actively engaging in it.

Walking Simulators.

Which is a parody style way of saying story games with no real gameplay like Home, Dear Esther, The Stanley Parable.

Good city builders/ management sims in general.
Good RTS (1 starcraft in ~4 years and 2nd was... argueble , total war is good, but doesn't really count, different scale and you spend more time in TBS).

GOOD WARHAMMER 40K GAMES!!! (not a genre but still)

Games like Jak and dexter/Jak 2/Jak 3 or Ratchet and Clank. I never to none sees games like these. Man... I wish more games were like that. Now days, mostly no games focus on the way like those games did.

FPS TD. How often do you see these games anyway?

Real time strategies with base building.

Hack & Slash TPS (like DMC series and Metal Gear Rising)
Real time tactics without base building (World in Conflict)
Stealth games

Turn-based strategy with elemental combat like in Crystal Warriors for Game Gear. (yes, I am that old). You have a party of characters that each have an element that is weaker and greater than another. Like: Water beats Fire; Fire beats Air; Air beat Water.

SPS games. In other words, Second person camera shooter games.

I always wanted to see a game, or rather a simulation, where I define the enviroment parameters and a simple organism and see how it evolves. A serious approach to Spore you could say.

I'd like to play more real 3D RTS games like Homeworld series!

Rhythm games in general

Base building big-map strategies. The last good one was Supreme Commander: Forged Alliance, back in November '07. Square Enix fucked up SupCom 2, so that series is dead (Planetary Annihilation looks like SupCom 2 but with many little maps in one game), EA killed the C'n'C series with RA3 and C'n'C 4:TT, Generals 2 turned into a browser game, Cossacks died with Napoleonic Wars, Homeworld is stuck in limbo (though I'm hoping the HD remake of Homeworld 2 doesn't touch the gameplay), Age Of Empires 3 was meh, Starcraft 2 was/is too micro, End Of Nations became a fucking MOBA,...