Spoiler alert :
To transfer the five metahumans to Lian Yu, Barry needs Snart's help for backup. However, Snart asked for something that Barry immediately said it's impossible to get or to give to Snart.
What did Snart ask of Barry?
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Leonard probably asked for powers (like meta humans). In the new 52 comics The Rogues placed their themed weapons into Elias' genome recoding device, and fused their DNA with the weapons. Captain cold then gets the power of Cryokinesis. Leonard probably wanted Barry to make him into a meta-human, so he could be more powerful.
Sorry, im not 100% sure.