Luxray - Electric/Dark.
It's a carnivore. It's mostly black. It's repeatedly stated to be extremely vicious when fighting. It learns Crunch, Thief and Snarl, and can be bred for Night Slash. And it's an extremely cool 'mon. Being Dark wouldn't help it much competitively, but it would certainly help its reputation.
Garchomp - Dragon/Fighting or Dragon/Dark.
Face it, the only reason why it's Dragon/Ground as of now is to give it a 4X weakness without resorting to the overused Dragon/Flying combination (being able to fly doesn't exactly scream "cave dwelling monster" either).
With either of these types, its ferocity and fighting capabilities would be recognised further. It would lose that sweet Earthquake STAB, and would probably have to go looking for a different primary ability, but it would lose its 4X weakness and gain STAB on... uhhh... Brick Break or Crunch, I guess...
Nah. I guessed Garchomp's secondary type was the subject of many discussions over at GameFreak's before they settled for this.
Conkeldurr - Fighting/Rock
Because concrete. And a scary combination of STABs. Perhaps it would be too similar to Terrakion, which is why I propose:
Terrakion - Ground/Fight
He doesn't have too many rocky attributes, to be honest. And I guess he would enjoy Earthquake STAB while missing out on Stone Edge.
Cloyster - Water/Steel
With its mighty defenses, and zero resemblance to anything icey, Cloyster really deserved the Magnemite treatment when GSC came about. Unfortunately, it stayed Water/Ice, which I guess it learned to appreciate when Icicle Spear was invented, but a Steel retype of Barrage would have done just as well.
A Poison immunity, cancellation of weakness to Grass and Rock, a bunch of new resistances and canonicity of its impenetrability would have made Cloyster slightly better off defensively (the tradeoff would be a Ground weakness). I don't know how the jokes about its resemblance to lady parts would change, though. Is it better to be a steel you-know-what than an ice you-know-what? Anyway, with Shell Smash and that typing, Cloyster would have been the physical tank of the century, taking hits while dishing out loads of pain.
Also, here are Competitive Retyping I and II, for those interested:
I - http://www.smogon.com/smog/issue10/retype
II - http://www.smogon.com/smog/issue11/retyping