It's possible, it's just not very likely to happen. There've been LOTS of these threads to pop up over the past few years and the answer tends to remain the same: it's extremely unlikely to happen for several different reasons.
1. WWE & TNA Are In Competition - Competition isn't a bad thing, but TNA is currently competition in name only when it comes to viewership. From what I understand, WWE significantly outdraws TNA in terms of television audience size in every country they both air in, with the exception of the UK. Even then, WWE programming, if I'm not mistaken, airs at 2 or 3 am in the UK and airs on a pay channel if I'm not mistaken, where as TNA airs in prime time on a standard channel. The only ones to gain anything from cross promotion is TNA. They'd have the opportunity to increase the size of their audience, to get their name & brand elevated to a higher level all at the expense of WWE. Because of that alone, it's almost certainly not going to happen unless Vince comes to someday own TNA.
2. Too Many Egos - WWE & TNA in a joint promotional endeavor would almost certainly have waaaaaaaaaaaaaay too many cooks in the fire trying to add their own little recipe to the mix. The ego of everyone from the WWE & TNA brass to the egos of the creative team to all the way down to the talent are certainly going to get in the way. In all those areas are people that have massive egos and I just don't think it'd work. For example, I have little doubt that Vince would want his company to ultimately come out looking like the superior one. I'm sure Dixie Carter, Jeff Jarrett & Hulk Hogan probably feel the same way regarding TNA. Again because of WWE's far superior audience size, you'd have to wonder why Vince would EVER agree to allow a rival wrestling company outside of his personal control to even look like they're in the same league as WWE. There'd be no logical reason I could think of for Vince to elevate TNA at the expense of his own company.
3. Public Perception - For the vast majority of the past 5 years, TNA is a company that has built itself with, primarily, using former WWE and/or WCW talent in the highest profile spots in their company. The VAST majority of these former WWE and/or WCW talents are wrestlers that the WWE fired or have had no interest in having on their roster again. Because of this, TNA has developed a reputation as being a haven for the so called "WWE Rejects" and using their own younger, homegrown talent to keep those older veterans built up instead of the other way around. TNA has moved away from that philosophy in recent months, which is a good thing, but it's still going to take several years for TNA to develop their own talent in which a majority of wrestling fans might view as being on par with WWE stars. Here's an example of what I mean: Bobby Roode has been doing a good job as TNA WHC. He's good on the mic, has a great look and can go inside the ring. Now, from the perception of most fans, how do you think they'd respond to a match like Bobby Roode vs. John Cena? Most fans wouldn't take it seriously because Cena's star power completely overshadows Bobby Roode at this point. Cena has been one of the biggest names and biggest stars of the biggest wrestling company in the world for the better part of a decade. Not that it wouldn't be a great match or an interesting feud, but perception is reality in wrestling. A LOT of fans would have no idea who Bobby Roode is and that is a relevant factor whether anyone likes it or not. On average, remember, IW tends to average roughly 1.5 million viewers each week, which is about half of what SD! draws and is anywhere from 1/3 to 1/5 of what Raw draws at various points throughout the given year.
As I mentioned earlier, the only way something like this will feesibly happen is if Vince McMahon winds up owning TNA. That way, the only cooks in the kitchen deciding what's on the menu will be who Vince decides. Whatever takes place will ultimately be up to him & him alone in which he can get the kind of outcome that he wants and the outcome that he thinks will be most beneficial, first and foremost, to the WWE.
