These are the coins they consider adding:
Cardano, Basic Attention Token, Stellar Lumens, Zcash and 0x!
More info about them here:
https://www.cardano.org/en/home/
https://basicattentiontoken.org/
https://www.stellar.org/
https://z.cash/
https://0xproject.com/
Official Coinbase blog post:
https://blog.coinbase.com/coinbase-is-exploring-cardano-basic-attention-token-stellar-zcash-and-0x-9e44f0eb823f
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