The concept of Crypto Broker can be dangerous provided cryptocurrencies have key-based ownership. It would be very difficult for anyone to trust third-party with his/her crypto holdings. We need middlemen to ensure that coins remain safe even if third-person is trading them. Exchanges can solve this problem. Exchange can let someone else control the coins of depositors but keep the coins in its own custody and only depositor can withdraw them.
I think there is already something similar to your idea. I have seen it on WazirX and the concept is known as 'Smart Token Funds'. It allows users to give control of their funds to
smart traders who can invest funds in their own portfolio but can't withdraw the funds of users. This concept is more alike mutual funds wherein every
smart trader provide his portfolio and users can invest funds into the portfolio by buying units of his Smart Token Funds.
Detail about STF:
https://stf.wazirx.com/List of STFs:
https://wazirx.com/stf/XOLO-USDTI did look into this. The concept is quite similar to this but has the trading part has a major difference.
WazriX STF has tokens for every trader and the users have to buy those tokens inorder to give their money to the Smart Traders to trade.
The lack of trust is the mode important thing here and WazirX STF has done the same thing which I has thought.
It's using an exchange as a trust factor. My idea was the same. But still the trading part is yet to be implemented.
It surely is a big thing and will require lots of time and effort but I guess it has never been done before.
I think the idea can workout but how can I do such a big thing all alone without any business experience and investment.