Mueller already has a strong case for collusion with the Papadopoulos confession and other circumstantial pieces, as well as proof Sessions has lied to Congress at least twice.
The dots are not hard to connect. His final report (assuming he is allowed to finish his job) is likely to point to the Trump campaign's willingness indeed eagerness to receive assistance from the Russians.
Remaining points of interest:
1) Who else gets indicted and for what. The list of people in legal jeopardy is quite long and goes up to Trump and Pence.
2) Who flips.
3) Additional details of collaboration between the Trump campaign and the Russians.
4) What Mueller does with the obstruction issues that arise from Trump's interactions with and eventual firing of Comey. He could do nothing, indict Trump, or send a report to Congress. Again assuming he is allowed to finish his work.
5) Whether Mueller charges others (as he did with Manafort and Gates) for crimes not related or incidental (such as perjury and failing to file accurate disclosure documents) to collusion with the Russians but uncovered in the course of his investigation. Again the list of people in legal jeopardy is quite long. And the indictments of Manafort and Gates suggest that Mueller is likely to pursue such cases.
6) Whether the Congressional investigations turn out to be travesties or substantial investigations.
7) Whether the findings of Mueller and the congressional investigators lead to impeachment charges or Trump's resignation.
8) Related to the above is the political fallout, including results of the 2018 mid-terms.
That's a lot that will be keeping this thread active and interesting for at least the next year. But I think the basic question of collusion has been answered in the affirmative.