Recent developments have been boosting the profile of bitcoin and other cryptocurrencies, which are unregulated and highly volatile. Futures began trading this month, believed to be a way to draw professional investors and oversight. An ETF product would add further legitimacy.
"It's very hard for us, as currency analysts, to follow this," said Thin, who is Brown Brothers' global head of emerging markets strategy. "It represents further mainstreaming. Hopefully that what comes out of this: some more regulatory oversight."
"Beyond that, we don't have any calls on where it will go from here."
Despite bitcoin's notorious volatility, the cryptocurrency has soared nearly 2,000 percent in the past year, according to Coinbase. Cboe futures were down 3.6 percent Wednesday.
Like other ETFs, investors in the ProShares Bitcoin ETF would reap benefits when the value of futures contracts climbs.
"By being long Bitcoin Futures Contracts, the Fund seeks to benefit from daily increases in the price of the Bitcoin Futures Contracts," according to the SEC filing. "The Fund will not be benchmarked to the current price of bitcoin and will not invest directly in bitcoin. When the price of Bitcoin Futures Contracts held by the Fund declines, the Fund will lose value."
On the other hand, investors in the ProShares Short Bitcoin ETF would benefit when the daily value of the futures falls and, like the first fund, would not be directly invested in bitcoin.
Bitcoin fans have long wanted an ETF for the digital currency without much success. But turning that dream into reality appeared to become much more likely after two of the world's largest options exchanges — Cboe and CME — launched bitcoin futures contracts within the past month.
"I think it is going to enable finally the approval of bitcoin ETFs, and other digital currency ETFs, which is game changing," Barry Silbert, founder and CEO of Digital Currency Group, said on CNBC's "Squawk Box" earlier this month.
— CNBC's Evelyn Cheng contributed to this report.