Everyone loves spending free money. I'm talking about beers paid for with a crumbled $20 bill found in the pocket of a jacket you haven't worn since last winter. Or taking your favorite person out for a great meal with the winnings from a scratch lottery ticket. You could save this cash or pay bills, but c'mon, spending it is so much more fun.

Now, imagine someone handed you a check for $1.6 billion -- and the check didn't bounce when you deposited it in your bank account. What would you do with all that cash? 

This is the question Shire(SHPG) - Get Report executives get to ask themselves now that the proposed $54 billion merger with AbbVie(ABBV) - Get Report is dead. With the U.S. Treasury Department closing tax inversion loopholes, AbbViedecided acquiring Shire was no longer attractive. AbbVie must now pay Shire a $1.63 billion breakup fee, according to terms of the proposed deal.

Shire isn't obligated to spend AbbVie's money, but it's no fun speculating about earned interest. Which drug companies might be takeout targets for Shire?

Let's start with the orphans. Last year, Shire bought Viropharma to gain access to a new drug to treat hereditary angiodema. Shire folded Viropharma into a new business unit focused on treatments for rare diseases with $2 billion in annual sales. If Shire desires to make a big splash in drugs to treat rare diseases, it could acquire BioMarin Pharmaceuticals(BMRN) - Get Report . With a $10 billion market value already, buying BioMarin would be expensive, but the AbbVie cash could be leveraged like parents who give their kids money for a down payment on a house. A Shire-BioMarin merger would create a rare disease powerhouse rivaling the Genzyme unit of Sanofi(SNY) - Get Report

If BioMarin is too much to chew, Shire could seek smaller targets to grow its orphan drug business. Let's throw out some names, including the usual suspects often mentioned as takeover bait: NPS Pharmaceuticals (NPSP) Synageva Pharma (GEVA) , Hyperion Therapeutics (HPTX) or Ultragenyx Pharma(RARE) - Get Report

Shire is probably too risk-averse for something like this, but buying Duchenne muscular dystrophy drug makers Sarepta Therapeutics (SRPT) - Get Reportor Prosensa (RNA) in front of next year's expected FDA advisory panel and approval decisions would be an epic roll of the dice. It's free money, so what does Shire have to lose? On a somewhat related note: How about Shire buying Bluebird(BLUE) - Get Report to make a play for gene therapy? 

Here's another idea: Shire dives into RNAi therapeutics by acquiring Alnylam Pharma(ALNY) - Get Report , Isis Pharma (ISIS) or maybe a smaller deal for Tekmira Pharma (TKMR) . The former two may have too many existing drug development partnerships and collaborations to make a deal work but since we're only fantasizing here, why not? 

It's fun to spend free money, even more fun to spend someone else's free money.

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback;

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