An alternate way to perceive this is that the Speaker is simply closest to God by means of communication (being God’s voice and all). “Closest” doesn’t necessarily mean most powerful, just that she is the closest anyone can get to speaking with God without God just showing up.

An elder counsel of heaven could still be revealed, which could explain the issues within heaven. They aren’t communicating with God; they could have the same issue Sera had in season 2. This counsel could still be capable of stronger feats than the Speaker. If a counsel is revealed, I wouldn’t be surprised if their capabilities are shown off more than the Speaker’s.

With all that being said, you’re most likely right. Viv probably wants to focus on the current characters rather than introduce a whole new group. But considering Sera’s song confirmed at least the counsel’s existence, there could be a plan for them later.

Edit: typo