This type of thread has been posted many a time on many a forum and all it ends up being is a Cilan vs Brock (vs sometimes Tracey) thread. It involves the exact same arguments over and over. It is not interesting and promotes little actual discussion. I would recommend you make a more general thread if you want an actual discussion- more along the lines of "Which characters would you like to see in Kalos?" This off-hand comment, though, belongs more in GCD. I'm going to merge it into there.
And I'll answer- Brock coming back would be fun if they expanded his role. Less womanizing, more PokéDoctor in training. More interaction with other characters beyond the mains and the girls he confesses his love to. Make him a more well-rounded character and I'm for it.
But also you must consider a few other things- Cilan's lasted what, four American seasons? idk I don't know where the dub separated things. Tracy lasted thirty-four episodes as a main character. And that's when Brock wasn't nearly as established. Obviously Cilan doesn't suck that much. He's a fun character with an interesting knowledge base and varied interests. He's a competent battler and is obviously skilled in his chosen profession(s). He's been a bit shafted by unappealing writing and lack of character focus, though. I'd love to see him return in Kalos and have his character expanded upon as well.
But then again, what of the characters of Kalos themselves? We've got a huge character pool right from the get-go- Tierno, Serena, Calem, Shauna, and Trevor. Then Clemont and Grant and Sycamore and Alexa and Viola. As far as I can tell, it's not going to be necessary to bring any characters back besides Ash, Pika, and the TRio. Now that Brock has been replaced for a good long time, they are the only main permanents left in the anime. Perhaps the writers will take to replacing the group 'adult' every season as well from now on. Who knows?