Apparently, they got Batman to go all the way to China, risk his life, illegally kidnap a Chinese citizen who had not been convicted or even indicted on any US-based crime, and bring him back for questioning and they didn’t have an actual plan. They had NO IDEA what to nail him on, they just knew that he was the money-man for the mob. So, when he finally admits to being the banker for numerous criminal organizations, Harvey has a eureka moment with Rachel and blurts “we can get them on RICO!”, presuming of course they can prove just one of the organizations pooling their money with Lau had committed crimes.
A bit of education: RICO (Racketeer Influenced and Corrupt Organizations Act) is a US federal law. That means HAD that been the case that Harvey chose to pursue that path, he would have to hand over the entire case to a FEDERAL court, taking it completely out of his hands to prosecute – his jurisdiction was at a city level for Chicago (I mean Gotham). He was not a federal prosecutor. If he was, he would be referred to in the movie as a United States Attorney. If some want to argue semantics that he was a federal attorney and merely referred to as a District Attorney, that argument will fail because such positions are appointed by the President of the United States. They are not elected to that position, as is the case with Harvey Dent (it’s pretty clear in both the viral marketing and election context of the movie).
That was just one flaw with that segment; another flaw is the fact that Lau’s lawyer just stood there doing nothing while Harvey and Rachel were using unjust tactics to pressure him to talk, making implications they would place his life in danger based on how much he cooperates. Because, without those tactics, they didn’t seem to even have a clue of what to get Lau to admit to which might implicate the criminals they were after.