In general, "at will" employees don't have anything to negotiate. It's strictly take it or leave it. There are no contracts unless the CEO gets involved, and the CEO won't be getting involved just for you. The hiring manager will just pass on you and move to the next-best candidate.
You negotiate your starting salary, and that's it. So make sure you account for the likelihood that you will get zero notice and zero severance when you get fired. Which is to say you need to ask for more money, and save some of it in high-liquidity, low-risk form.
People wonder about salary inflation, but when you look at total compensation, it isn't quite so extreme. Companies cut or eliminate benefits, and employees have to demand higher salary to compensate. The salaries are going up at a higher than usual rate, because companies are slashing benefits packages faster than usual.
Those companies are figuring out the hard way that employees are not clueless about the value of their benefits. New employee candidates coming in will see the crap benefits and ask for more money. Existing employees will leave for either benefits more similar to what they once had, or for more money. In general, cutting the benefits results in being less competitive overall as an employer, because it usually costs less for a company to buy the benefits in bulk, than for employees to buy them piecemeal.
In the case of severance, the major advantage for the company doing it is that you absolutely cannot spend that money on anything else until you are fired. You could get that by buying a special insurance policy for unplanned unemployment, or you can discipline yourself to never touch your "severance fund" except in dire emergency. Also, you are taxed on a severance payment in a year when you otherwise have less income, because you got fired, whereas a personal "severance fund" is taxed as you earn the principal, and again when you liquidate, if you realized gains. Make sure the company will be paying for all that before you start work, because once you start, most of your leverage disappears. They no longer need to convince you to quit your previous job, and only need to do just enough to keep you from quitting your new one.
The last severance I got was in 2002. No company since has ever been willing to pay it out or even negotiate the possibility. Severance is dead. Long live the personal "oh shit" fund. Make your employer pay you enough to have one, along with a personal retirement fund, a personal vacation fund, a personal health care fund, a personal commuting budget, and personal equivalents of all those other things that many companies once paid for, but no longer do.
We don't need a union, if everybody can individually commit to asking for more money.