I found the following on the IRS website:  https://www.irs.gov/government-entities/form-w-2-and-form-1099-misc-filed-for-the-same-year

If you look down at the last example:

Additional Compensation Payments Reported on Forms 1099-MISC

When a taxpayer files both Form W-2 and Form 1099-MISC for a worker for the same year, and the payment reported on Form 1099-MISC can clearly be identified as additional compensation (such as a taxable fringe benefit), the examination of whether the additional compensation is subject to employment tax is a wage issue. The consideration is whether the payment meets the definition of wages or is excludable under a specific Code section.

Example 4

During an examination of the taxpayer's 2019 employment tax returns, the examiner found the taxpayer filed both Form W-2 and Form 1099-MISC for several workers. The taxpayer explained that the amounts reported on the Forms 1099-MISC were year-end bonuses paid to employees. Since bonuses were paid to workers for services performed as employees, the bonus payments are considered additional wages subject to employment taxes. The examination would not be considered a worker classification issue.

I've highlighted the relevant part.

According to this IRS website, a bonus paid to an employee is subject to employment taxes.  So, either the employer needs to include it as wages, include it on the W2, and pay the employer portion of the employment taxes OR the taxpayer reports it and pays the employment taxes. 

The third alternative is to file an SS-8 with the employee's tax return (and they pay their portion of the employment taxes).  Since there are always two sides, the IRS will then notify the employer of the potential misclassification.  There are some significant penalties for worker mis-classification so this route often ends up with bad relationships between employee and the employer and often results in termination of the employee before it is resolved.  While this is probably the "right" way to do it (if the employer refuses to change their W2), I typically explain to my clients the potential repercussions.

But it is clear that employment taxes must be paid on bonus income.  So, going the route of "other income" is not correct.  If the employee/taxpayer chooses not to request a corrected W2, then I will only prepare their tax return with a Schedule C and self-employment income.