What was the plan for season two of Limitless?

There are not a lot of specifics, but...

From Executive Producer Craig Sweeney:

Sweeney previously revealed that he has big plans for "Limitless" season 2 but it may not focus on Brian Finch (Jake McDorman) any longer after his big secret was revealed unless there will be drastic changes. "I think that unless something were to change fairly drastically, there would be no place for him there," he told IGN. "His utility to them is as somebody who can take NZT without the harmful side effects... so Brian's time at the FBI is over."

AND Speculation from ScreenRant:

Limitless essentially took its literally mind-altering concept and turned it into a procedural, which arguably wasn't the most creative direction to go in. Limitless season 2 would have seen the public aware of the NZT drug and Morra going missing, which would have changed the whole dynamic.

That said, the crux of Limitless season 2 apparently would have still involved Finch solving crimes, so it wouldn't have been a major shakeup. Morra had also announced his candidacy for President, which no doubt would have been a major subplot. Unfortunately, with Netflix and other platforms having long since passed on it, season 2 looks very doubtful.