Currently NFTs is still trending and I don't think that trend has lasted a long time, and therefore it might be hard knowing the next big thing after it. From what I can deduce when the trend slows down, another idea will pop up, now this idea might not be a new one but might be an existing one which will gain attention and then massive developments will start on it; this is just like NFTs, they have been around for a long time now, but no one really paid attention until recently when improvements and so on were done.
Nevertheless, I have started seeing some people now working on layer 2 technology, although it has been around for a while now, but some developers are now trying to enhance on it and offer a unique idea altogether. This is just an observation though, but until the NFT trend slows down, then we can know the next one.