I think a lot of what people have said here about Sayaka is solid, but I'm interested in how Touko might respond to Yuu's confession. A lot of people seem to think that because she seems like she's starting to become happy with herself, that Nanami is likely to accept Yuu's confession, but remember that that wasn't the original problem.
Touko thinks that love is about expectations and restrictions. She even viewed her own love for Touko as contingent on Yuu not feeling love for anyone. It's not just an issue of self-loathing, or even "they don't really love me", it's several layers of misconceptions all built up on one another. One of the key roadblocks is that she loves Yuu because Yuu has no expectations of her, but she doesn't realize that Yuu's unconditional acceptance of Touko is Yuu's love for Touko.
Now, it's possible that that all unravels when Yuu confesses. It's possible it all unravels if Sayaka confesses. She might actually find that when it comes from someone that she really trusts, she finds it easier to accept their feelings. (after all, the other confessions have been from people she barely knows, given over superficial reasons that didn't feel like they connected to her real self.) But that doesn't mean things go smoothly.
There are actually a lot of things that could go wrong. Sayaka might decide to confess, just to put an end to her feelings (I don't think she'd try to subvert Yuu, but she might just do it for her own sake, expecting to be rejected) and Touko might accept, thinking she's imposed on Yuu too much.
Yuu might confess, and Touko might retreat because she doesn't know how respond to that change in their dynamic. It's even possible she might take it as a betrayal, an attempt to impose restrictions on her at a time when she's still struggling to find her identity. I could see them separating for awhile over something like that.
Of course, if either of those happened, I'd lay odds that Sayaka would be the one to give them a push back together. Or maybe I'm just totally off base, and the next phase of the story is about Yuu and Touko learning to develop their mutual feelings together. I just wouldn't expect things to go that smoothly unless we were right at the end of the story.