The most feasible potential offerings in my opinion are .coin, .wallet and .crypto the others are probably too limiting for ex, .ico would be the equivalent of a burner because it would only be useful prior to and during an ico sale. Extensions like .BTC, BCC, .Ethereum might be too limiting because most companies use and will use multiple currencies like monero, dash, litecoin etc.
They might work but I feel like the actual usage may be limited to (THIS PRODUCT/OFFERING ONLY).(Currency) for example Exchange.BTC or Loan.BTC, Crowdsurance.Ethereum etc...