It would definitely take an amazing new set of features to replace Bitcoin as the top cryptocurrency in the world.
It would be something other existing crypto-currencies doesn't have and something future crypto-currencies cannot beat or even copy perfectly.The only real possible scenario where an altcoin could overtake Bitcoin is if there is some serious flaw in the code that basically causes bitcoin to stop working.
It is believed by a good part of the community that bitcoin will be killed by another cryptocurrency and not just simply replace it. Bitcoin would be dead so they we have no other choice but to use a different cryptocurrency or use the coin that killed bitcoin itself. Almost all altcoins promise that they are better than bitcoins but I don't think they actually are since no one has surpassed bitcoins yet. 