Sundays are for Disney:
All of my ideas exist purely in the realm of fantasy because Disney clearly has no time for or interest in runDisney events that occur outside of the cookie cutter event window they apply for all of their race weekends. There was a time, I believe, that making rD events unique experiences was important to them and it was about more than filling rooms, restaurants, and parks. Unfortunately, I don't believe that is the case anymore. I believe it has even turned the corner to the point that the events are a bit of a nuisance to them since they actually seem more interested in depressing attendance than increasing it during much of the year. Don't get me wrong, I still love Marathon Weekend and when I ran Dark Side in 2017 I had a fantastic time as well; the CMs and volunteers are still amazing and rD still does a good job with hydration and their course setup and entertainment. It's just, especially for the price, I'd love to see them trying new things, exploring different race windows, and considering different ways to make things surprising even for people who have done multiple races.

Oh well, I'll get off the soapbox, they're still getting my money and that's the ultimate contributor to their complacency.
There are any number of themes that would be awesome for an rD event. There are very few prospective events that wouldn't interest me in the off chance my money tree grove ever establishes itself. Personally, I'd love to see a Sugar Rush 5K or 10K with the Wreck-It Ralph crew, K-Pop blaring through the speakers, and some colorful/crazy decorations.
On a more general level, Disney has been doing a lot of special ticketed events that either give guests early access (Toy Story Land ride/breakfast thing, etc.) or late access to the parks (Disney After Hours, etc.). I think it would be a really cool if they offered events like these exclusive to runners over race weekends. Obviously, with the price of their races, including such perks in the price of registration wouldn't be out of line (although I can't imagine they would't monetize such offerings).
Ultimately, I'd just like them to try new things, put races in different time windows, and differentiate the experiences again.