Further centralization of mining
It is quite possible that mining may become more centralized. In the summer of 2016, the block reward will halve to 12.5 BTC; this development may move some of the smaller miners into an unprofitable territory; hence, forcing them to stop mining. As a result, the big miners may end up with a bigger overall percentage of the hashrate. Manufacturers of mining equipment constantly put out better, faster, and more efficient hardware. There is a consolidation happening in this space; thereby, creating larger mining entities. Quantum computing may also introduce a miner with enormous hash-power, and of course, if the BTC value, as measured against local currency, skyrockets, then it will encourage more investment into mining equipment from large established mining firms.
Hardware arms race
In order to mine profitably, your hardware needs to be better than that of your competitors. Currently, Bitcoin mining ASIC manufacturers are in a kind of hardware arms race, trying...