The episode "Run Silent, Run Deep" features Louie--it's towards the end of season two.
I will shamelessly admit that there can never be too much Danny drama for me. If every episode focused on Danny grappling with a case, trying to mend his relationship with his brother, trying to get over Ruben's death, trying to decide whether to give it up for Flack or not
...I'd be a happy camper. He's hands down my favorite character, and I think whenever Carmine does get those storylines tossed his way, he does a great job with them.
That's not to say I don't want more Hawkes, Adam, Sid, etc. but given the choice...well, I'm pretty much going to pick Danny every time, because I do enjoy the character's complexity so much.
Really, my dream show is a Danny/Flack spin-off. No, not that way. (OK, yeah, that way
but I'm not sure even HBO could do them justice.) I'd love to see a buddy cop show, or a cop and the CSI show I guess, with those two. I like most of the other characters (well, all save one), but the relationship and chemistry Danny and Flack have just doesn't have an equal on the show, or any of the CSI shows really. I could watch those two snark suspects all day long.
I'm drifting OT, so I'll bring it back to what I want to see in season 5: Danny, Danny and more Danny.