Trump may declassify requested FISA documents
Declassifying documents so the general public can see them renders the FBI attempt to protect those documents moot, and plants a massive egg on the face of everyone involved -- including the intelligence community actors who have so far received relatively little scrutiny, and possibly (if not in these documents, then in later batches) on the political actors. If Trump starts declassifying things, that opens a can of worms that probably can't be closed. All the schemes would be exposed and the information out in public.
That's good for Trump, assuming he's innocent of the Russian collusion charge. That's why Mueller has been desperately digging for something else he can use as leverage. He's nailed a few people for tax issues and rarely-used registration laws, flipped Michael Cohen for more tax issues and got him to plead to a probably-invalid campaign finance crime, but it hasn't given him any real leverage on Trump. And it's Trump he needs leverage on to avoid the declassification scenario.