I think that the further they got on with new Pokemon, they did pretty good with most of the typings, but looking back to the first gen Pokemon in general, there are quite a few I can see that really could've used a re-typing at the same time as Magnemite/Magneton. Just taking a quick glance through the Kanto dex, I would've like to have seen:
All the Nidos as Poison/Ground (not just the final evos), Ninetales as Fire/Psychic (always thought it was for the longest time), Golduck as Water/Psychic (again, always thought it was), Seel/Shellder as Water/Ice (just like their evos), Cubone/Marowak as Ground/Normal (80% of their moves are currently Normal type), and make Kangaskhan Normal/Ground as well.
Taking all those into consideration, though, I didn't really consider if there were other Pokemon that are already of that type or not, but I've never really seen any of those I mentioned as just single-type Pokemon. There are probably a number of others I could've found sprinkled throughout the other gens, but like I said, I think they've mostly done a good job of those typings, which just a few that could've used a change.