I would like to make lore and style clarifications on the pokes already suggested in The Smog retyping articles. I see some of them fitting while others not so.
I -
http://www.smogon.com/smog/issue10/retype
Bronzong -> Steel/Ghost
There is nothing lore-wise sticking Bronzong to Psychic typing that Ghost can't do (ie. floating and portals). In addition, its ancient and mysterious lore and style would fit the Ghost typing.
Gengar -> Ghost/Fighting
I personally have no idea why Gengar was given Poison typing. I remember wondering as a kid why Ash had to get a Ghost to beat Sabrina when the Ghosts in the game were actually we to Psychic thanks to its Poison typing. Fighting fits Gengar much better because of its more tangible form as it evolved and that it learns powerful physical moves like Drain Punch and Giga Impact.
Starmie -> Water/Electric
I didn't know how I felt about the change until I looked up its information. I am actually shocked Starmie isn't classified as Water/Electric already. Everything thing about Starmie's descriptions and information elude to Electric. It rainbow colors involving the light wavelengthes, the flickering of its core like a lightbulb, and most interesting, the sending of electrical signals into space. Another O_o to Gamefreak on this one.
Metagross -> Steel/Ground
I don't think I can find a way to get rid of its Psychic typing. So much about revolves arounds its ability to float on command and its extreme intellect.
Yanmega -> Bug/Dragon
I see where he is coming from, but Yanmega doesn't learn any Dragon moves and the only Dragon it can mate with is Flygon which can mate with several Bugs. If anything Flygon should be the candidate for that typing. Dragon Yanmega is a bit too much of a stretch for me.
Staraptor -> Flying/Fighting
Yes please. Only Flying type that can use Close Combat, sounds like a match to me.
Heracross -> Fighting/Rock
I can't stand for this one. Its literally a Bug >.>
II -
http://www.smogon.com/smog/issue11/retyping
Claydol -> Ground/Poison
Another typing that just can't happen. It would naturally lose it ability to Levitate.
Alakazam -> Psychic/Ghost
I can see it, but yet another stretch I personally don't feel comfortable with. Sure it can Teleport and learn Shadow Ball, but its too much of a stretch.
Wheezing -> Poison/Electric
Like Starmie, I was unsure until I read the pokemons background. It is stated on Bulbapedia that is has the ability to use the gas particles within to generate heat and electricity, so why not the typing for it.
Mismagius -> Ghost/Dark
I don't see a reason not too. It looks the part and it will benefit from it.
Azumarill -> Water/Fighting
Azumarill begs for this typing. It's ability Huge Power and the powerful Fighting moves it can learn, I am surprised it isn't Fighting already. Even fits evolition-wise as the baby form having Normal typing, it showcases the growth of the pokemon.
Absol -> Dark/Psychic
Absol would appreciate the Psychic typing lore-wise thanks to it keen intellegent and ability to foretell disaster.
Ambipom -> Normal/Dark
A cheeky monkey that can learn Dark moves, sounds like a nice fit.
III -
http://www.smogon.com/smog/issue22/retyping
Serperior -> Grass/Dragon
Serperior just begs for a noble typing and what is more noble and majestic than a Dragon typing. It already learns Twister and is tutored Dragon Pulse and Outrage. Give Serperior the love it deserved.
Darmanitan -> Fire/Fighting
If this typing was already in the game, Emboar would be kicked to the curb. This pokemon fits so well with this typing being such a massive Fire physical force. Why hasn't this happened.
Archeops -> Flying/Ground
As amazing as a typing it would be for competitive reason, it just can't fly lore-wise. Every other fossil pokemon Rock typing. Sorry, but I don't feel that can be broken.
Zoroark -> Dark/Ghost
Similar to Alakazam, its a bit of a stretch to add Ghost typing, but it actually would fit for it. Zoroark is the Illusion Fox pokemon and being able to shift itself like that and the spirtual nature of foxes can make the Ghost type work.
Beartic -> Ice/Water
I appreciated the new typing choices made for B/W creating the first Grass/Normal and Flying Mono, but I fell the did Beartic an injustice making him a Ice Monotype rather than just slapping on the Water typing like in the past. It a polar bear that knows Water moves. Just why not?
Cryogonal -> Ice/Steel
I could really see this working. It having the ability to learn Flash Cannon and the samurai look really hints to the very interesting idea of a Ice/Steel type.
Gothitelle -> Psychic/Ground
I see no relevance for this lore-wise, Psychic/Dark however I can. Being able to outlive several trainers and being able to see their trainer's life span would make anything a little darker.
Mienshao -> Fighting/Normal
I totally don't see why they didn't give Mienshao the Normal love they gave Sawsbuck. Mienshao doesn't look like a traditional Fighting type and its moveset would allow the typing.