It seems like the three main developments happening in currencies involve privacy, decentralization, or scaling.
Which of these developments are going to see the greatest growth in the next upcoming months?
sadly these two are not related!
the "growth" among altcoins is based on pump and dumps. believe it or not the biggest growth always belongs to the most pumped or most manipulated altcoin. they even have as big as 5000% rise in one month. that is ridiculous amount of profit. so if you want "greatest growth" or biggest pumps then you have to find such coins and invest in them before their start of pump.
the development should logically be the reason for a rise or growth but since we are talking about altcoins, in reality it is not the reason although it helps the altcoins in long term. for example good altcoins such as Monero that is the best privacy coin stays up in long term and even grows because of it. but you can't expect big pumps like %5000 from these coins. specially since their prices are already high.