You took the words right out of my mouth. I suspect that the ultimate smoking gun will be money laundering because, as you noted, Trump was doing that for years. I suspect that is the only thing that kept him going through all the bankruptcies until the magical Deutsche Bank funds became available. And those arrived, apparently, only with Russian guarantees.
The Russians helped in the election for the reasons you cited and because they have been working on election tampering for years learning from the US. And Trump or his son were stupid enough to get caught asking them for help and that is another smoking gun but it is almost too stupid to impeach him over. It was an accident that he won and neither Putin or anyone else knew what to do with it then. A classic case of a bunch of people, some of them very dumb, making various moves independently that suddenly blew up on them. It happens all the time just not at that level.
Mueller has too much to work with but the US tradition is to use tax evasion or money laundering to take down the big guys. Conspiracies are purely made up for the media that has no interest in anything but entertainment.