Currently, there are 40 unused type combos.

[ItWasABadDream]

I want a fire/poison type so that I could demolish it with an earthquake >:)

 

[Sabrewulf238]

I can actually foresee a Normal/Ghost idea being plausible. Like maybe a pokemon based off the ghost of a human, lol.

What about a pokemon where some parts of it's body is ethereal while other parts are solid?

Or perhaps something like Parasect with a ghostly being possessing an innocent animal.

Personally I'd love to see Steel/Poison, Psychic/Poison and Electric/Fighting.

 

[alisaie]

A Fairy/Ghost would be pretty insane to have. Three Immunizes and two weaknesses (steel and ghost) which no pokemon has a type combo of.

 

[fenyx4]

The abundance of unused type combinations involving the Normal and Poison types is staggering; the presence of a lot of unused Fairy type combos is to be expected, I guess, due to being newly-introduced.

Hey, thanks. That's actually the exact thing I was going to link to.
But personally, I'd love to see some of these combos. A Bug/Dragon type would be awesome, even though I'm not much of a user of either. There are also a lot of good puns that could come out of these - a Grass/Rock (yes it has been done before, but still) acoustic guitar could evolve into an Electric/Rock electric guitar. Just for pun.

Great pun. XD

A lot of the normal types wouldn't really make sense though. So it would be stupid to have those types added. Like a normal/ghost is a complete oxymoron!

There's some peculiar Pokemon design just begging to be given that type combination, I imagine. XD

Well, there you go. My username is close enough for a Water/Fire type (And comes with a unique range of signature moves too ;)).

As for new typings, I'm surprised Electric/Fighting isn't a used combo. If I was developer I'd re-categorize Electivire's typing to that new typing. We know it has a high Attack stat, so why not? Ice/Fighting is another combo I think will be coming up in Gen 7, although the only possibilities would perhaps be something like a fighting penguin/polar bear if we were to take that chance.

{XD} Electivire does lend itself quite well to the Electric/Fighting combination, although, if Magmortar is perceived as its counterpart, I'd want Magmortar to be dual-typed as well... But I'm really not in the mood for more Fire/Fighting Pokemon at the moment after what happened between the fiery starter Pokemon of Generations 3, 4, and 5. -.- An Ice/Fighting penguin Pokemon would be cool, although I would hope that the design is distinct enough from Empoleon.. The Pokemon Garnet fangame had a great looking penguin Pokemon that might've worked well... Also, we could have an Ice/Fighting "snowman" Pokemon.. XD

I'd love to see a grass-dragon type. It'd probably look really cool, and I'd expect it to do alright gameplay-wise despite the 4x ice weakness.
Am I the only one disappointed that Ninetales didn't gain a part Fairy-type?
Also, Normal-Ghost would be incredibly powerful. Immune to Fighting, Normal, and Ghost, and weak only to Dark.

I wanted Serperior to be Grass/Dragon (what with it being all regal and serpentine, as well as a starter Pokemon), but nooo... I could see Ninetales as part Fairy-type, helping to distinguish it from the Fire/Psychic Delphox..

Fire/Grass! Been wanting this type combination since the original games haha. I think it would be SO cool to have and definitely the one I am rooting for most next gen :D

I know, right? I would love to see Smogon's Fire/Grass Pokemon Pyroak become an official Pokemon to finally manifest this type combination! ^^

Surprised Electric/Dark hasn't been done yet - a lot of people mistook the Shinx line for that typing, anyway.

The Luxray line definitely should've been Electric/Dark; we have a good amount of pure Electric-type Pokemon as it is. Wasted opportunity... >:(

I could imagine the Fire/Poison typing to be a good starter, and, I desperately want a Bug/Dragon! Yanmega could become it if it gets a mega, just please Nintendo, give us a beefed up Bug.

Fire/Poison scorpion starter Pokemon FTW! :P I've wanted Bug/Dragon for the longest time, but it seems like (lately) we repeatedly get a Pokemon who could be this type combination but ends up not being it.Maybe a Mega Yanmega Mega Evolution someday...?
Generation 3 - Flygon (Dragon/Ground who manages to Levitate)
Generation 4 - Giratina (Dragon/Ghost), Yanmega (Bug/Flying)
Generation 6 - Zygarde (Dragon/Ground...again)

Phantump, Trevenant, Pumpkaboo and Gourgeist exist. They have the same typing, though it's reversed. Doesn't make much of a difference either way.

Besides the rumored Fire/Water Volcanion, I'm looking forward to a Fire/Grass Pokemon, as other people already mentioned; I hope that Game Freak makes something like Smogon's Pyroak a reality.

Woo-hoo; another Pyroak supporter! {:3}

I'll forgive the lack of combinations using Normal or Psychic. What I am most displeased with is the lack of a Bug/Dark type! PLENTY OF BUGS ARE SCARY OR EVIL

Fire/Water should have happened in a funky gen like Hoenn. Hah. Now we just have some weird steam Pokemon.

LOL; the Fire/Water Volcanion would have been great debuting in Hoenn.. XD And yeah, Bug/Dark needs to exist as a Pokemon already.. >_> The Fakemon Naoie (although rumored/conceptualized as Dark/Steel) could work well in that mold, too, being a bug-ninja-samurai...thing. XD

Im not too mad about the lack of Normal/X pokemon, because it would kinda make it not normal right?

That's how I felt, too; I just hope that more Normal/elemental Pokemon are as offbeat as Bidoof and Bibarel are. :D

---
So, yeah; wishlist being:
Fire/Grass: Pyroak
Grass/Dragon: Serperior
Fire/Fairy: Ninetales?
Dragon/Fighting: Fakemon Jakaroo - The Pokemon Garnet fangame had a boxing kangaroo named "Jakaroo"; I thought it would be very neat to implement a kangaroo as a 'dragon' (long-ish neck and tail; vaguely similar to Goodra's shaping) with this unused type combination, because it would just be unexpected and so jarring for some people. XD

I also want more Pokemon that are the Rotom types (as in, I think all Rotom forms should be elemental/Ghost rather than Electric/elemental, so I want other Pokemon to use up the Electric/elemental type combinations that Rotom currently uses).

 

[Legobricks]

Plot twist: A spin-off of the games where all of the contradictory type match ups (Like the Normal/Ghost oxymoron mentioned above, Fire/Water and Fire/Grass, etc) are mashed together and turned into Pokemon that upset the balance so you need to catch them all. Maybe as a Pokemon Ranger?

That's most likely to end up being the concept of the legendary monsters in a future main-series generation.