Bitcoin will always rule all the altcoins because altcoins' exchange rate will always be related to bitcoin exchange rate.

Since most of the altcoins don't have really strong support on exchanges – by that I mean they cannot be traded directly to other altcoins – it is difficult to say that one altcoins will be powerful enough to displace the King of Crypto – Bitcoin.

I hope though, that bitcoin dominance will still decrease since more money will be distributed among new blockchain projects with their own cryptocurrencies listed on exchanges.