Quite frankly, I find it silly that you would consider Twilight Sparkle the "leader" of the six. While Twilight Sparkle may be the most prominent figure of the group, she certainly isn't the "leader". If anything, they are all leaders. They all balance each other out and guide each other. They all have unique strengths, weaknesses, and personalities, and thus are all leaders.
However, in the absence of Twilight Sparkle, I would assume that their friendship would continue to grow, and that they would all guide each other in more ways than they already have.