Let's talk about the reality of debt (from an economics only standpoint, no politics today).
If you're in debt, you can still buy things. In fact, you can (quite easily) go into more debt, especially if you have collateral. Obviously it'll bite you in the assignment eventually, but hey, if you ever gathered enough stuff when the debt comes due, maybe it's not a big deal.
So here are two variants that try to mirror reality a bit better.
Variant 1: Bubble Economy.
While you have debt tokens, you may accrue more debt, and you may buy cards. You cannot win the game if you have debt tokens when the game ends.
In this variant, you should probably get rid of your debt before you buy victory cards.
Variant 2: Gordon Gecko.
While you have debt tokens, you may accrue more debt, and you may buy cards. At the end of the game, debt tokens are worth -1 VP each.
Bonus variant: Interest. At the start of your turn, gain 1 debt token for every 8 debt tokens you have. Only usable with the above variants.