Most severance packages are negotiable, and most people don't negotiate. The five things to push on (in rough order of company flexibility): Accelerated vesting. If you have unvested equity, ask for additional months of acceleration. Companies often grant 1-3 months as a goodwill gesture, especially if your departure is performance-neutral. PTO payout vs. extended termination date. If you have unused PTO, ask whether they'll convert it to extended employment (preserving health insurance) instead of paying it out as a lump sum. This is often easier for the company to approve than cash. References and re-hire eligibility. Get the reference policy in writing, including a named contact (HR or your manager) authorized to respond to future reference calls. Re-hire eligibility status (typically yes/no flag in HRIS) also matters, push for "yes" unless you actively don't want it. Outplacement services. If they're offering, ask whether you can take the cash equivalent instead. Most outplacement firms are mediocre; cash is more useful. Mutual non-disparagement language. If they're asking you to sign non-disparagement, push for it to be mutual. Many companies will agree because it costs them nothing.
What you usually CAN'T move: severance multiplier in a mass layoff (it's standardized), benefits continuation length (legally bounded), and stock option exercise window (board-set).
Quiet rule: the "calm, written, single-page email" gets you more than the angry phone call. Always.