I know you've done a tremendous job running this for a number of WCs now, so it's not my position to suggest changes ........
Having said that, I'd like to see a change for selecting the "winning result" after the group series are complete.
To get 2 points for selecting Belgium to beat Panama, or Russia to beat Egypt in the group stages, seems disproportionate to getting the same 2 points, when tossing the coin between say England v Belgium in the later stages.
Off the top of my head, people got around 50% of the group results right, and after the group stage, this fell to around 40%.
Maybe as the correct score bonus goes up, the correct result points could increase a little, too.
I do note, it probably would have made no difference to the monetary results, if you had done that this Cup.