There's a lot in the bits and pieces of evidence we have, even if there's no slam dunk evidence, but only arguments from probability. The FBI has a lot of evidence they aren't going to share until the investigations are over.
The dossier on trump the ex-mi6 agent made (which is certainly full of a mix of reliable and unreliable sources since it was a raw release that wasn't vetted) is kind of mind boggling. External validation of some sources and reports are validating more and more as reliable. There were a few of reports in the dossier created around Jun that were predicting things like Carter Page getting a payoff with a 19% of Rosneft's value, provided the Trump admin would work to drop sanctions, and criticize NATO allies for not paying their fair share, and a few other things, where later a sale of that 19% value was made, and Trump began parroting that language after that predicted sale. It's all circumstantial, but the coincidence is so epically unlikely, and the Feds could get/may have much more solid evidence on what was going on with the communications, sale, and details on what was going on during Page's repeat visits to Russia. Russian agents tied to the dossier are mysteriously dying in Russia, which also adds to the sense that it's not all some elaborate 4chan fiction, though it'll take actual investigator to get solid evidence rather than what now might be an incredibly unlikely coincidence.
From what bits and pieces we have, everything points to Manafort seeing some trouble since he was on the Kremlin's payroll (for which there's solid evidence) and worked to change the GOP's platform on Ukraine as Trump's campaign manager. Carter Page (who was visiting Russia, and quoted as belligerently criticizing liberal democracies while praising Putin, and who's tied up in the dossier) being toast, and Flynn hopefully suffering more due to his attempts to cover up many connections/contacts with Russia, Turkey, et al. There's even a chance that Stone's reported repeated (and badly covered up) direct communications with a Russian intelligence front, combined with repeatedly predicting future document dumps by Russia look like there's probably enough for him to be in some trouble.
Hard to say what'll happen, but there's data out there besides the reports of intelligence agencies, just as there's a lot of data tying the DNC/Podesta hacks to APT28/GRU besides the claims by intelligence agencies.