Just the top 50 on CMC is good.
thats exactly the problem, this thinking will one day end you, the ground under your feet wil disappear suddenly as soon as people define new markets, in new regions etc.
I don't personally really care about new coins in crypto, most of them are shitcoins in my opinion. I like sticking to the regular popular ones, tron, eos, btc, eth, xrp. The top 50 coins on CMC would be a good metric to base the platform's payment system on, but of course there are other great coins that are not top 50 on CMC, just don't know of any right now.
just put escrow systems. you can go try look on bitify.com. with escrow system will give you much more people to come when they will feel safety to do trade/transaction. make sure fees is not too high and fast withdrawl.
Seems like a good idea, incorporate your own escrow system or use another company's.