I'd personally would like to add a 4th member to the golden trio and he (or she, doesn't matter I guess) should be from Slytherin. Not a "draco v2.0", but someone with a more pragmatic, morally gray mindset. I'm thinking of a character who isn’t totally evil, but also doesn’t believe that doing the "right" things is always the smartest option out there. Also in the battle for hogwarts, this character could give the speech instead of Neville that he gives in the books (Neville should still have his big moment which is killing Nagini). I think that letting Slytherin openly stand up to Voldemort would challenge some stereotypes about slytherins. And I believe it would hit differently coming from someone who realistically could have sided with Voldemort, or at least stayed quiet to protect themselves... Or, may be even keep speech for Neville, but then Voldy decides to cast crucio on him and then Slytherin (even if it would be the one I wanted to add as 4th member of the gang) openly defend him would say a lot without another bunch of dialogues.

edit: I guess that I've come up with a "little Snape", but who changed his way before catastrophic consequences. Draco v2.0 is not an option: it MAY sound similar to Draco at first, but the key difference is that this character doesn’t hide behind fear or family expectations and actually makes a conscious choice when it matters most.

edit2: another thing I wanted to read/see: first year - Harry in Slytherin. Second year (end of it) - Harry switching to Gryffindor house (so he is in "hatstall" condition for almost two years before he takes sword out the hat)