He won't be able to go to TNA, he'll have the same clause in his contract that Lesnar did. If he cuts and runs early, the no compete clause will remain in effect until the end of what his contract would have been. So he won't be able to go to TNA even if he wanted to and they wanted him, because his contract was a multi-year deal, which he had barely scraped the tip of. So no wrestling in the U.S.A for Lashley for a while. However I think Lashley will like so many others have stated, either quit and return to the WWE after realizing, he won't make as much money anywhere else. Now he will quit, because even if he changed his mind, he is smart enough to know, that he has still really pissed off the powers to be, and we all know what happens to the ones who really piss off the powers to be... they get jobbed, buried, humilated. Lashley won't want that happening, so the only option the poor bastard has open for him right now, is to quit.
Now on the off chance, that TNA is an option, I think for starters and I agree with others, than TNA needs another show to spread out their talent and they need another title. Now back on track, I think both TNA and Lashley would be good for eachother. TNA would give Lashley the chance to truly hone his skills and become the well rounded superstar that lies within, while not dealing with all the pressure that comes with being a WWE superstar, and TNA will get, well lets face it, a animal, Lashley is tough, he is explosive, and would be good at the mid-card level to help elevate the whole mid-card status. But unfortunately for both TNA and Lashley, I don't see that happening.