As much as everyone is saying Cena and how much he's a cash cow, he's a cash cow because of his appeal to females and children. TNA is TV-14, built for a mature audience, Cena's Hustle Loyalty and Respect have no place in the impact zone. I don't think Hogan's 80's character wouldve worked in the Attitude Era and I don't think Cena's character will work in TNA, unless he returned to the Doctor of Thugonomics
That being said, let's bring up someone who actually is the 2nd best mic worker in all of professional wrestling. Let's bring up someone who truly is the 'complete package' more than anyone in this generation of sports entertainment. The only straight edge champion in the history of the WWE, CM Punk. Punk has been a success in every company he's been in, and TNA would be no different. His feud with Jeff Hardy in the WWE was one of the best of Hardy's career and Punk could either continue this rivalry or start new ones in TNA.
Punk is also the same age as Orton and Cena, although he is not making as much money right now (let's keep in mind he's been injured and before then he was getting buried on Smackdown) he's going to be around for just as long as is more talented than both men in my opinion.
In regards to your Cena comments, I'd say that Cena still has a great number of adult male fans. The bulk of his fans are women and children, but in no way does that mean he doesn't have a sizable amount of adult males backing him. In regards to your choice, CM Punk is an incredible worker, and he'd probably do well in TNA. However, you seem to be ignoring some factors in this scenario.
Who could help get TNA on the map and bump it's credibility?
CM Punk doesn't have the drawing power of many guys in the WWE. He has the support of the IWC, and that's about it. A bigger name and IWC darling, Kurt Angle, didn't propel TNA to greatness (though it did bump its credibility up), so I don't see how CM Punk would draw any more than their existing former WWE stars.
Who could work well with the current TNA roster?
CM Punk
would work well with the current TNA roster. He's a versatile guy, for the most part. I can't fault your choice in this area.
Who could earn TNA more money?
CM Punk wouldn't bring in as much money as a top name like Cena, Orton, or HHH by a longshot. Again, just the support of the IWC doesn't mean mainstream support, and mainstream support is what translates into money. Punk isn't the most marketable guy, especially when compared with the top WWE stars.
CM Punk definitely isn't the best choice, in my opinion anyways. He'd have great matches and cut great promos in TNA, but there are other people that could do that while still drawing viewers and revenue.