First of all, all this "home grown" talk is nonsense. I say it every time that stupid fuckin' buzzword is thrown around as if it has bearing on the realities of professional wrestling because it's a total cop out and a line of thinking that's completely full of shit. WWF proved it. WCW proved it. How? By getting guys who weren't "originals" to their respective companies over as fuck and by making MONEY off them, which is the primary (and only) goal of professional wrestling in the first place. If "home grown" had any bearing whatsoever in this business, explain the resounding success of Steve Austin, Hulk Hogan, Mick Foley, etc. etc. etc. etc. etc. all of whom spent considerable amounts of time working in other companies before they found their success in WWF.
Second, if I'm picking one of TNA's non-ex-WWE talents to be their next "breakout" star, the answer is the same even if I was picking from a list that included ex-WWE stars. Austin Aries is the answer, regardless. He has everything required to be a major starthe look, the style, the gimmick, and more. The only thing working against him at this point is age, though it's not a factor in determining whether he will be a star, just in how long he'll remain one.
Robert Roode and James Storm would also be among the contenders here, though at the moment no one is rising faster than Aries.