Coming up with an Ultra Bonus is more or less a matter of inspiration. However, there are a few things to keep in mind when writing yours.
First, there are typically three clues given to the ultra at the top of every hour. Obviously, you should make the clues progressively easier as each hour passes. The 8th set of clues should all be well-known examples of the theme, and the theme should be the first thing you'd expect someone to think of when looking at them.
The case used to be that all ultra clues were verbal clues. However, in recent years, visual clues have become more common, and on at least two occasions there have been audio clues as well.
While all of the clues should clearly relate to the theme once you know what it is, an individual set of clues can have its own, misleading sub-theme; for example, pictures of Shelby Manhaten, C. Montgomery Burns, and Nelson Mintz from "The Simpsons" were the fourth set of clues for the "US Senators" Ultra.
A list of all known Ultra Bonus topics to date can be found here: https://wso.williams.edu/orgs/trivia/ultra.html