My idea on how Slytherin can be represented in the new series

Whilst Slytherin was initially conceived to be the bully house, I don't think it's a particularly popular portrayal of the house nowadays. Speaking as somebody who is a Slytherin, this is what I would do to make the Slytherin more realistic and believable, whilst staying as close to the source material as possible.

First of all, I'd make a big distinction between Draco Malfoy's year and the older Slytherin students. Malfoy becomes a lot less interesting to me if he isn't the biggest racist of Slytherin house. The way I'd develop this distinction would be through the Slytherin quidditch team. In particular, Marcus Flint, Adrian Pucey and Graham Montague.

Flint I would characterise as a light hearted brute, and really play up his lack of intelligence. A man with no skill at actual quidditch, and is constantly relying on his sheer strength to get through the game, usually committing fouls in the process. An idea I have for an additional gag would be to make him constantly forget who is on his own team.

"You've got no chance this year, Wood. Daxtor will catch the snitch for us in seconds again, just you watch."

"Marcus, Daxtor isn't on the team this year..."

"What!?!? Why not?"

"He graduated from Hogwarts at the end of last year. You replaced him with Higgs yesterday."

".....Oh."

I'd make Pucey and Montague highly serious and disciplined students, who put a lot of attention on their craft. Pucey with his academics, Montague with his physicality (a joke a came up with would be him doing pullups on his own broomstick). Both of them would clash nicely with the Weasley twins, constantly mocking their abilities as beaters.

Overall I think this would help establish the Gryffindor and Slytherin rivalry, without painting the entirety of Slytherin house in a bad light. But that's not to say I'd not have the completely evil Slytherin all together. I would save it for when we get to Order of the Phoenix onwards. When Lord Voldemort returns to full power and Draco Malfoy grows in both age and influence, that is when we should see the students from Malfoy's year downwards call everyone mudbloods and blood traitors. It'd help set up the darker mood that the later books had.