The new set of players could be people who never participated in BTC at all - futures contracts could make it much easier to short on a large scale for those who would rather not go on dodgy exchanges like Bitfinex.
The largest current BTC exchanges are still nowhere near the level of professionalism that most rich people who might want to try manipulating the market would expect.
It makes it easier to go long as well, but it was definitely harder to bet against the price of BTC rising than in favour of it before, and I expect futures contracts to change that at least.
The new set of players are professional traders. They have no interest in manipulating the market, they just want to make a profit. If it looks like its going up they'll go long, if it looks like its going down they'll short it.
I think the whole "whales manipulating the market" thing is coming to end for this very reason. Whales are about to getting eaten by some sharks.