/vp/ won't shut up about fairy-type. Basically, it's half "fairy-type no es fake" and half "fairy-type is stupid". While fairy-type sounds really stupid, we have dragon-type, so "type based on specific thing" isn't unheard of.
If GameFreak does introduce new types, two things need to happen.
1: Make it weak to poison. As it sits, only grass is, and grass is paired with poison a lot, nullifying that weakness. I have no idea why bug isn't weak to poison. Or water. Or fighting.
2: Don't make it weak to dragon. While dragon is only super effective against dragon, that's by design. The dragon-type houses powerful Pokémon, and making everything weak to dragon would be silly. And by silly I mean retarded.
Another few types only have two strengths, those being electric, ghost, steel, psychic, and dark, so giving those more strengths would be nice. Grass could also use a buff, since rock and ground are covered by water (which you need anyway), and electric covers the rest of what grass covers.
Other underappreciated types, like bug and ice, are really good types. They just aren't given to many good, easily-obtainable Pokémon.
People are also saying that the aforementioned fairy-type would be retconned onto existing Pokémon, ala Magnemite and steel. One of the Pokémon they use as an example is Mawile. I believe that giving Mawile another type would be beneficial in many ways, but only if it resists fire, fighting, and ground. Coupled with the weakness to poison that the new type should have, this would give Mawile no weaknesses, just like its counterpart, Sableye. Sableye and Mawile don't have much in common beyond their counterpart status from Generation III. This would make them both lack a weakness.
So, a new type could be great if it worked well with existing types, and was presented in such a way that it seemed like something we should have had to start with (like steel and dark).