Frankly, I don't think it's gonna happen...since they gave it a new bro in Heracross right away... And with Scyther gaining no stats with evolution, its much more like a forme than an evolution..except its one-way.
What I'm trying to say is, Scyther can still be regarded as an end-stage Pokemon. Its not like every other pre-evolution which you just wanna evolve. So Pinsir and Scyther are still meant to be "compared", if that's the right word.
But I wish Pinsir would get one anyway, because its new bro, Heracross has this unfair extra Fighting type, which basically makes it much closer to Scizor in usefulness, while Pinsir is barely ever used even though its stats are 500 total like the other two...
As for the type it would gain.
Steel? No, if they were to do that, it wuld have been with Metal Coat along Scizor.
Fighting? Eh, that would make it more like Heracross than is healthy.
Nope, what Pinsir already looks like it could gain is....the other very special type...that was introduced alongside Steel: Dark.
But lets get realistic. The best one could hope for Pinsir is to gain a Bug type Quick Attack move called Beetle Smack or something xDDDD
Actually, anything is possible! It doesn't have to always be traded with a Metal Coat that would turn a Pokémon into a Steel-type. Think of Leafeon for instance, it didn't evolve with a Leaf Stone, it evolved while leveling up next to a moss covered rock in every games prior to Diamond/Pearl/Platinum. ;)
It would be cool to see a new evolution of Pinsir. And who knows? It could end up having wings and exoskeleton covering it's wings like the beetles found in real life, and would end up appearing in a similar form to Heracross, but much more metallic. :D