I think it's really hard to find another big thing like bitcoin because Bitcoin is always the best for investment. If you still want to find out, i suggest altcoin. I can try to learn about it
On my opinion I think that, people who invest in bitcoin on the year before bitcoin was known for having a cheap/low price and we should start investing from now. And I myself believe that in the near future bitcoin will have a much more expensive price than the price now. Thus, you can see that by doing a research by researching it and get every single information of every coin on the blockchain and on that, you can think next big thing like bitcoin. But right now the only coin that I think become very high in the near future is the Etheruem.