The most popular is BTC/USDT or ETH/USDT and you also create your own token too. But I think, you should make some different thing from other already existed exchanges. I'm working hard with my team to build an exchange too, it takes a long time, but it has new policy for investors
I think creating a new exchange is not a good choice at this time, but if you have a new idea, I encourage you to do so. Most of the exchanges are currently experiencing a lot of liquidity issues that make these exchanges last only two months. I am waiting for what you do during this time