Citizens United established that corporations, as legal persons, are entitled to free speech, including spending unlimited amounts of money in political "speech".
Under Constitutional law, CU asserts, corporations and individuals are equivalent in their civil rights.
Therefore Federal and state tax treatment of corporations and people must necessarily be equivalent--"equal protection under the law", via the 14th and 5th Amendments.
That means that for tax purposes, individuals are corporations.
In particular, individuals, since they are corporations for tax purposes via the implications of Citizens United and related precedents, may only be taxed on their income after expenses related to the production of income. As corporations are only taxed on their gross income net of "cost of goods sold"--which includes most major corporate expenses such as employee salaries, training, benefits, rent, materials, supplies, freight, utilities, and insurance.
But virtually all personal expenses are related to the production of income--they are the "cost of services" that an individual sells to their employer. So individuals must be allowed to deduct all their living expenses (food, clothing, rent, mortgage, supplies, furnishings, utilities, insurance, transportation, medical, education, interest, etc.), and will only owe tax on their remaining net income in any given year. (If any!)
Citizens United implies that the personal income tax is unconstitutional if a more liberal tax treatment is provided to corporations. Equal protection under the law!
When courts uphold the above "doctrine of tax equivalence", it will precipitate a dramatic tax revenue crisis overnight. The IRS will be required to change immediately to the new all-corporate system. 300 million newly corporate-equivalent citizens will suddenly be able to achieve zero or near-zero tax liability. As corporations do now.
In that case, you can be sure that those in charge will exert furious efforts to eliminate "corporate personhood", by Amendment or by any other means, with the utmost haste. In fact, it would likely be resolved as a national security emergency, by Executive order, until an Amendment could be ratified.
The "flash" tax rebellion starts... now! Class action! Citizens United will be our shield.
(Disclaimer: the above is not legal advice--consult a lawyer before asserting either your corporate personhood or your personal corporate-hood on your tax return!)