Fire/Dragon - Come on Charizard. Why couldn't you have just been this typing instead? It makes more sense. Past mistakes aside, it's just one of those combos that you know would work: it's very logical and it's hard to imagine the end result looking anything less than overwhelmingly cool. Competitively, fire/dragon is a good offensive duet, and getting STAB on both Overheat and Draco Meteor is great. Add Levitate as an ability (come on, I know it's been mentioned thousands of times in this topic already, but can you not imagine a Fire/Dragon without any sort of wings like Charizard's or Salamence's or the ability to levitate ala Gyarados or even Volvagia from OoT?) and you're good to go.
Steel/Ghost - More of an lol suggestion than anything. Just the sheer oxymoron of a ghost that's as dense as steel is enough reason to make it. It's also not a terrible defensive combination, with the both types having a good amount of resistances and immunities, as well as Steel negating the weakness to Dark and fellow Ghost attacks and Ghost granting immunity to Fighting. Once again, tossing Levitate in (I swear to god that this is the last time this ability's going to come up for me, but it's a ghost, and it would just add to the oxymoronic-ness of having something as heavy as a Steel-type floating in the air) would leave it with, to the extent of my knowledge, only a weakness to fire. I would love to see Bannette with an evolution like this.
Grass/Ghost - http://tn3-1.deviantart.com/fs17/300W/f/2007/201/7/f/Jakolan_by_RockoTheTaco.jpg. I know it doesn't seem very ectoplasmic, but it's a start. We can make this happen people! It's more of a fun type though. I don't know how practical it could be.
Fire/Steel defensive type - Torkoal is just begging to become a competent fire-type Rapid Spinner (and a competent defensive fire-type Pokemon...oh hell, a competent Pokemon in general) with this new typing and boost in stats.
Dragon - Is it too much to ask for one fully evolved dragon to be a pure-type? Apparently, with Roost's invention, that became too much for Flying. I see nothing of the sort stopping Dragon.
Dragon/Ghost - And not one that looks more like a goddamn centipede than a dragon. I was thinking something along the lines of a skeletal dragon, akin to Warcraft 3's Frost Wyrm. A second thought...
Ice/Dragon - Much better.
Poison/Dragon - http://tn3-1.deviantart.com/fs17/300W/f/2007/196/a/a/Komodra_by_RockoTheTaco.png. I'm pretty sure this is the last of my Dragon-type suggestions. This makes for a nice change of pace though, one for being the first dragon-type that's not imposing or majestic (which is a cool thing actually) and two because Poison needs more love. Seriously, some of these designs kill most 4th gen Pokemon designs.
Fire/Electric - This screams out Manectric. That 4x ground weakness hurts, but I think the ability to actually harm most water types not named Swampert with a STAB TB is incredibly sexy.
Ghost/Anything would be pretty neato torpedo now that I think about it, so long as it's not a reused type.
Fire/Water - http://img.photobucket.com/albums/v292/TheUltimateLifeForm/MrVolcano.jpg. I'm filling my water with volcanicity! Just seeing something like Mr. Volcano as a Pokemon (which is the only way I can see a Fire/Water turning out) brings a smile to my face, practical or not.
Flying/Fighting - http://tn3-1.deviantart.com/fs18/300W/f/2007/181/e/3/Brikait__d_by_RockoTheTaco.png. Farfetch'd evo. I think enough has been said.
Steel/Ghost - More of an lol suggestion than anything. Just the sheer oxymoron of a ghost that's as dense as steel is enough reason to make it. It's also not a terrible defensive combination, with the both types having a good amount of resistances and immunities, as well as Steel negating the weakness to Dark and fellow Ghost attacks and Ghost granting immunity to Fighting. Once again, tossing Levitate in (I swear to god that this is the last time this ability's going to come up for me, but it's a ghost, and it would just add to the oxymoronic-ness of having something as heavy as a Steel-type floating in the air) would leave it with, to the extent of my knowledge, only a weakness to fire. I would love to see Bannette with an evolution like this.
Grass/Ghost - http://tn3-1.deviantart.com/fs17/300W/f/2007/201/7/f/Jakolan_by_RockoTheTaco.jpg. I know it doesn't seem very ectoplasmic, but it's a start. We can make this happen people! It's more of a fun type though. I don't know how practical it could be.
Fire/Steel defensive type - Torkoal is just begging to become a competent fire-type Rapid Spinner (and a competent defensive fire-type Pokemon...oh hell, a competent Pokemon in general) with this new typing and boost in stats.
Dragon - Is it too much to ask for one fully evolved dragon to be a pure-type? Apparently, with Roost's invention, that became too much for Flying. I see nothing of the sort stopping Dragon.
Dragon/Ghost - And not one that looks more like a goddamn centipede than a dragon. I was thinking something along the lines of a skeletal dragon, akin to Warcraft 3's Frost Wyrm. A second thought...
Ice/Dragon - Much better.
Poison/Dragon - http://tn3-1.deviantart.com/fs17/300W/f/2007/196/a/a/Komodra_by_RockoTheTaco.png. I'm pretty sure this is the last of my Dragon-type suggestions. This makes for a nice change of pace though, one for being the first dragon-type that's not imposing or majestic (which is a cool thing actually) and two because Poison needs more love. Seriously, some of these designs kill most 4th gen Pokemon designs.
Fire/Electric - This screams out Manectric. That 4x ground weakness hurts, but I think the ability to actually harm most water types not named Swampert with a STAB TB is incredibly sexy.
Ghost/Anything would be pretty neato torpedo now that I think about it, so long as it's not a reused type.
Fire/Water - http://img.photobucket.com/albums/v292/TheUltimateLifeForm/MrVolcano.jpg. I'm filling my water with volcanicity! Just seeing something like Mr. Volcano as a Pokemon (which is the only way I can see a Fire/Water turning out) brings a smile to my face, practical or not.
Flying/Fighting - http://tn3-1.deviantart.com/fs18/300W/f/2007/181/e/3/Brikait__d_by_RockoTheTaco.png. Farfetch'd evo. I think enough has been said.