WWE did a cross promotion Raw with ECW. Stop acting like Vince has never tried to do something with a smaller company.
I doubt this will happen simply based on WWE ignoring everything that's not a part of the Universe anymore, unless its one of Ric Flairs 16 recognized title reigns.
Yeah but when they did that ECW crossover, the battle with WCW was still very close and VInce knew that if he didn't do the partner up with Heyman, then WCW would and he woulda lost out on the bump in ratings that bringing those guys in gave him (I realize it was probably a very small bump, but a bump nonetheless). That crossover was purely a move against WCW. If ECW was the number 2 promotion at the time, there is no way Vince would've gone for it... Hell, I'd guess that if they were even a closer number 3 he wouldn't have.
WWE would have absolutely nothing to gain from a TNA crossover nor would it counter any rivals move since they have no real rivals.... The TNA/ROH makes far more sense and I'm also surprised it hasn't happened already.