Instant answers are a huge must for people who use search engines as much as I.
As someone from a programming background, there's several "utilities" which would assist in the quick searches that are required day in and day out.
Some examples of useful calculators/converters/utilities which aren't currently in Kagi:
Colour value conversion
Example query: #ff0066 to rgb
This should be understood as a hex to RGB colour conversion, where ff0p66 is the hex input string, and RGB is the required output type
Example result RGB(255, 0, 102)
Attached is an image showing how DDG tackles this. They also show an example of the colour, as well as other potential value types (hsl, etc..)
Another example would be Unix timestamp manipulation.
Things like "current timestamp" which could display a live updating timestamp, and also other datetime related utilities, such as what will the time be in 4 hours from now (or more simply, time in 4 hours). These might seem like small, and often mundane things to question, but I believe that is at the heart of what to expect from instant answers - a quick way to know an answer with confidence for things we tackle all day.
More to the programming side, there's already a large array of great instant answers provided through stack overflow, however sometimes an official resource for a question is more suitable.
For example, when searching for javascript array prototypes, providing resources in summary from Mozilla's MDN would likely be more of use.
(Also, providing results from Wolfram alpha would he super cool, wouldn't it?)
