If there is any other alternative i will be in support of, that will be Waves and NEO. I think these two have the potentials and they are gradually coming up, but saying that they can totally substitute Ethereum is a no to me. I think ethereum will continue to be relevant and will be used by most projects. Notwithstanding, others will also gain more relevance as time goes on.
Waves could might be a rival to Ethereum though there is not that many people using it. Namecoin is another coin that could compete with Ethereum eventually.
How could waves be a rival with ETH while if we look at the list Waves are been much far adopted by anyone, Maybe we could look for its closest neighboor listed on coinmarketcap since those one are great coins with great volume. But there are huge chances or monero or even Dash to get covered if people will find more reliable one to hold or use with.