This is more what I'm thinking of:
Ruffalo's hair is not bad in that picture above, but his hair at the moment seems longer and slightly messy. Just cut it a little shorter or perm it a little tighter and he'll be giving off more of a Bixby vibe.
I just hope he isn't too laid back in the role though. He needs to maintain the nice guy feel and sense of decency, but Bixby also had a real intensity simmering under the surface. I'd like to see that in Ruffalo, so that you'd feel that you don't want to cross him or make him angry. If he seems to laid back, easy going and phlegmatic, you'd wonder how he's ever going to turn into the Hulk. It would be like "hey, don't worry about it, it's all cool".
Imagine how electrifying each scene would've been if Bixby were alive today and it were him in the Avengers alongside Downey Jr, Hemsworth, Evans and Jackson. Can you imagine the tension from the verbal sparring and conflict between the characters that would cause Bixby's temperature to rise?