In theory with the Sanders/Clinton split in the democratic party, and the giant fissure Trump is in the process of creating/expanding in the republican party, there would be room for more. It would be great if they both let themselves split and we had four parties.
Unfortunately the entire system in the US is rigged against that happening, and the fear of a split while the other side doesn't mostly forces them to kiss and make up when it comes time to cast a vote.
The 2018 party primaries may be "interesting", but the elections themselves probably mostly won't be. The risk for the republicans is if they primary sitting congressmen and senators who the hard right feels are unreliable RINOs and replace them with someone independent voters can't accept, and they give up seats unnecessarily as a result. The democrats may be on their way towards the same sort of thing, but haven't quite reached that point yet.