Hi Jennifer!
While the Rivers of Light Dining Package is only currently available through the end of June, I'm sure it won't be long before they extend the dates through the summer months. Unfortunately, we don't have access to any information about when future dates may open up, but I encourage you to keep an eye on the
Rivers of Light Dining Package page for future openings. What I've done in the past, when waiting for dates to open up for a dining event, is set an alarm on my phone at the same time each day to remind me to check the Walt Disney World Resort website! Hopefully you won't have to wait long, as I know Guests are eager to book this dining package for their summer vacations. Other great sources of the most up-to-date Walt Disney World Resort theme park information are the Disney Parks Blog or Walt Disney World on Twitter.
What you may want to do is go ahead and make a dining reservation for either Tusker House Restaurant or Tiffin's for the day you're planning to visit Animal Kingdom. Once your FastPass+ reservation window opens, you can then make a FP+ reservation for Rivers of Light on that evening. If Rivers of Light dining packages become available, you can always secure one at that time, and cancel your prior reservations. This exact scenario happened to our group for my most recent visit - I made sure to secure the dining package reservation first, then simply canceled the other!
We dined at Tiffin's Restaurant as part of our Rivers of Light Dining Package, and it was amazing. Not only was the meal delicious and the service outstanding, but our reserved seating section was dead center in the outdoor amphitheater! We had an amazing view of the gorgeous Rivers of Light show - I loved every second of it.
I hope this helps with your planning, and that you'll soon be able to reserve your dining package! - Diane W.