My hope is they will not be acquired and will keep this stand-alone as a true alternative rather than be eaten and then to lose all their advantages. Acquisitions of companies like this usually spell the end of the line. Here's to hoping Stripe will IPO and will go on to acquire who could now potentially acquire them.
Card issuers (VISA, MC), Amazon, Microsoft, major banks, Ebay, Paypal, Due, Payline, Paysquare. A leveraged buy-out would enable many more potential takers.
Google, dozens of international companies (eg Softbank or SAP), Walmart, Target, Costco, IBM, Oracle, and numerous others.
Basically, several dozen companies are serious potential buyers. And that's before getting into the secondary list, which includes companies like Intel, AT&T, Verizon, Comcast, etc.