What other bad guys could have been lingering in the shadows of that scene? Maybe a Connally shot someone else? The only person we know did not get shot, was Walt, as he shares the last scene with the owl.
Where the series turned south, INHO, was by not using one of the prettier cards in their deck, the last season, Lizzie.
Had they played out a three way sparks angle between the two women in Walt's life, that would have given the series more sizzle. They left this role completely out of season 3.
They also penned out of the story line, the romantic involvement between Branch & Katie. In making the series more clerical watching Ferg move from desk to desk, and less personal, they got off the track a bit of what made the show pop. IMHO.
I hope the series gets picked up by another network. The more time goes by, the less likely that is to happen.