Price manipulation is a fairly big threat already. There's already the
significant possibility that USDT (and Bitfinex) are a house of cards, and price manipulation could be occurring from a variety of different means in such an unregulated market.
Introducing futures while the market has still not matured much could make the market vulnerable enough for just one or two whales to pump and dump the BTC price to some crazy extremes.
There's very little to keep the price pegged down within a reasonable range, which could lead to euphoric speculation. And it's dangerously close to when that will be possible.