Currently if a formula field will return only a date or a number, Airtable will have formatting options for that formula field. However, if the formula will return a string or might return a string, or might return anything other than a date/number, there are no formatting options.
Sometimes I write a formula so that it will usually output a date or number, but will return a string if an error condition is met. For example, a formula might calculate the number of days between two dates (a number) but output a string if the end date is before the begin date.
This product suggestion has multiple parts to deal with these situations:
Allow formulas to output errors which are distinct from the calculated value. For example, if a formula calculates a negative number, but negative numbers are not allowed from a business logic point of view, have the formula report an error that would be recognized as an error, not just another value. This would be similar to the #ERROR message, but could include a human readable string saying what the error is from a business logic point of view.
Once formulas can report errors as errors, many formulas that sometimes return strings could then be considered to always return a number or a date or an error or blank, and thus could have formatting options.
Increase the formatting options for formula fields to include formatting options for all the formula types. For example, a formula could be formatted as a centered check box (not just a left aligned emoji), as a rating with stars, as a select (with the colors of a select).