I think the successful altcoins will be the specialized coins. These coins are created with clear differentiator and purpose. I don't think ETH will be successful. It's super bloated and has even more vague use cases than bitcoin. Dogecoin is a good example. The idea is silly but it has a clear differentiator. All other limited supply/store-of-value coins will eventually be chewed up by Bitcoin.

If you cross out limited supply, I think there's opportunity. I'm working on one: https://bitflate.org/.