Now the mainstream currency, BTC-ETH, has the upper hand, and they have the strength and technology, but the future is likely to be a new project to replace them more mature and more comprehensive. What do you think it would be?
i think no digital currency can replace bitcoin's in near future, it will be take a lot of time and lot of work for a new project to be better than bitcoin and this will be not easy task for new projects.
Bitcoin is the topmost currency of the crypto world and will even remain the top currency for a long long time. I do not think that there is any other altcoin which can compete with bitcoin. I think that bitcoin will remain as number one crypto currency for a long time.