Yes, that was one of my first concerns over Riku's role in KH3. I was worried that Sora and co. will have a lot of spotlight. But now that we see a conflict over Aqua falling to darkness and Replica Riku returning, Riku may have a big importance in this game. Riku may have to convince Aqua or help her from completely being consumed by darkness. Riku had also been in the influence of darkness in KH1 and it may prove that Riku could be the perfect person to help, this would also kinda tie in that Riku is a keyblade master as well as Aqua. Hopefully Riku successfully helps Aqua, because I feel that even though Aqua told Mickey, "You're too late", I feel that it isn't fully too late. That there has to be a chance, even if it is slim.
And honestly for Replica Riku, I can't seem to figure out about what importance he may hold. Possibly helping with the ongoing conflict in the series? Maybe he'll want to help Sora because of being in good terms with him at the end of KH: CoM or Riku when they sort of were okay with each other before Replica Riku disappeared? I honestly cannot think what Replica Riku's importance is, and it makes me really curious and really excited to find out.
As for personal struggles, I feel that him possibly helping Aqua and/or Replica Riku may serve as helping Riku with personal struggles. It seems to happen on occasions like in DDD. And whatever he encounters may also help strengthen him to, may teach him things as well. I'm trying to think and it's 9:35 pm. xD