Workday, which provides cloud-based human resources and other software for enterprises, seems to be most likely, according to RBC analysts.
"Not only does Workday, today, have a completely complementary product set relative to Salesforce's (thus potentially preserving his relationships by not competing directly with his former employer), Workday offers, in many ways, a similar situation to what Keith Block inherited when joining Salesforce seven years ago," RBC's global equity team wrote in a note.
Salesforce said Block will stay on as an advisor for a year.
Block could also return to Oracle, though RBC said it was "exceedingly less likely." After more than 25 years with Oracle, Block left in 2013 to join Salesforce as president and vice chairman.
"The opportunity to re-position Oracle as a cloud infrastructure and application software leader would potentially put Keith in the same conversation as Satya Nadella in terms of corporate turnaround master-strokes," according to the note. "We believe that there is a rare opportunity to heal old wounds."
"With Microsoft Azure competing more aggressively and taking share in the IaaS market, we think Amazon Web Services could be well served in adopting both an increasingly verticalized and application strategy that is more familiar to Fortune 500 enterprises," according to the note.
Shares of Salesforce sank 3% on Tuesday after the company announced Block will step down as co-CEO, with Marc Benioff to transition to the sole chief executive. The stock fell about 4.1% in early trading Thursday as part of the broader market drop.