Cancer has impacted my family as well, and you and your daughter have my sincerest condolences on having to deal with this at such a very young age.
I think my thoughts on this are…
Let your daughter decide where to go and what to see. Purchase recent guidebooks for Disney and Universal, and go through them with her. Make it your bedtime reading! Having unlimited Fastpasses will make it possible for her to design her dream vacation.
Consider experiences. Something like Bippidy Boppity Boutique or the Pirate League, if she likes dressing up. The Pirate Cruise or the Princess Tea Party. These can make her feel very special.
Be prepared to be flexible. What if she gets tired? Dropping everything you’ve got planned and heading back for a nap should always be an easy, low-stress option available to her. She should never feel that she’s disappointing anyone, or missing out on things, if her energy flags.
Take lots of pictures!
Edited to add: I really hope your daughter gets her remission, and that the trip can go ahead as planned. But do also take a moment to consider what you will do if the trip can’t happen, for any reason. Cancer is one of those awful, unpredictable things where you’ll need to have several back up plans in your pocket at all times. So, make those plans. Best case scenario, you won’t need them.