It does not have to be a "smoking gun." It does not have to be audio of Trump on the phone with Putin saying "Hey Vlad, let's collude."
The question to me is whether Trump knew Russia was taking action to help him win, that Trump overtly or even impliedly offered to assist in that effort, or whether there was any sort of discussion of helping Russia should he win, i.e. relax sanctions.
We know that Trump publicly invited their help and that his team met with Russian agent to "get dirt" on HRC. We know that polling data was shared with Russian agents. We don't know if there was any other effort to coordinate anything with Russia. We also do not know if there was ever an agreement to favor Russia once elected. The closest we have to that is the change in the GOP platform.
Based on public reporting, Mueller investigated how that change came to be. That,. to me, is of paramount importance. It will tell us a lot, one way or another.