I think Ethereum is definitely one of the altcoins worth investing in. In the past, I did not find it right to invest in any coin other than Bitcoin for the long term, but when I saw the thousands of developers working on Ethereum and some of the needs of the industry, I understood that it is definitely a valuable asset. Also, I think the important thing here will be the distribution ratio of your portfolio. If you think you have enough Bitcoin, it wouldn't be a bad idea to buy some Ethereum. 
Bitcoin and ethereum are heavy correlated, which means that when bitcoin goes up in price ethereum does the same, and when bitcoin goes down ethereum also follows it, so you do not get any protection by diversifying yourself and investing in ethereum or any other altcoin, for that reason if you really want to achieve some sort of diversification in your portfolio then you need to be prepared to invest in markets other than this one.