Um, I may be completely wrong here, but I'm pretty sure JK Rowling said something about how bloodlines and family actually are a factor as to determine where one is sorted? 

For example, people from the same families tend to be in the same houses*, and since Slytherin valued blood purity, that's also an influence. Not to say that everyone from the same family will be in the same house and that no one who's halfblood or muggle born could end up in slytherin, but I'm pretty sure it was mentioned by JK Rowling that it was a factor.

Personally, I say Gryffindor works best as well, but if you think Slytherin could work, well, you're GM, and there is that factor. I'll try to find the source of this info if it's iffy. It's been a while since I've last read up on this...