This may just be me, but I have never been all that impressed with JT Miller's game, and I personally think Oscar Lindberg has just as good of a chance of making the team out of camp as the 3rd line center.
I think a line of Hagelin-Lindberg-Haggerty/Kristo would fit really well together.
IMO Lindberg's overall ceiling is actually higher than Miller's. He's certainly a better defensive player than JT at this point, and watching Lindberg, I really see the potential to break out offensively with the right linemates and in the right situation.
Any reason I'm missing why Lindberg hasn't been discussed nearly as much as Miller for the 3C spot?
I know Miller got the call-ups last season, but Lindberg's overall game seems a lot more mature to me than Miller's who seems defensively lost at times and has a habit (not uncommon among young players) of trying to do too much when he has the puck, ultimately turning it over.
I especially feel that if Haggerty or Kristo are going to be on the 3rd line as well (a legitimate possibility), then Lindberg as the center (along with Hagelin) would be a better fit because of Lindberg's great two-way play and defensive awareness as opposed to having two rookies with unpolished defensive games on the same line (Miller + Haggerty/Kristo).
Regardless, training camp should be extremely exciting this season.
My ideal lineup would look like this:
Kreider [1C] St. Louis
Zuccarello Stepan/Brassard Nash
Hagelin Lindberg Haggerty/Kristo
Glass Moore Carter/Carcillo
assuming one of Stepan/Brassard is moved in a deal to acquire that bonafide first line center. Whichever one isn't moved becomes the 2C.
ie: Brassard, Miller, McIlrath/prospect for E. Staal or J. Thornton.