Responding to "I'll see you in my dreams" can be a sweet and charming moment. Here are some good answers you can consider:
"I'll be waiting for you there, and I hope our dreams intertwine beautifully."
"I can't wait to join you in the realm of dreams, where we can create our own world together."
"Knowing that you'll be in my dreams makes me look forward to sleep even more."
"In my dreams or in reality, you'll always be on my mind."
"I hope our dreams become a magical escape where we can share adventures together."
"You've already filled my dreams with happiness, and I can't wait to see what unfolds next."
"Dreams seem brighter knowing that you'll be part of them. Goodnight."
"My dreams are lucky to have you as their guest. Until we meet again."
"Wishing for dreams to be reality, so I can hold you close once more."
"Dreams are a little taste of the future I want to build with you."
Remember to tailor your response to your relationship with the person saying it, and let your genuine feelings shine through. Responding with warmth and affection will surely make the moment special.