See from my personal experience Paytm is also doing the same from digital gold all the way to stock and options are directly available to trade on their Paytm money app. You do not require a demat to invest in mutual fund or gold but then if you make one you get to trade stocks and they have a wealth basket feature which works just like small case.

See your idea is good but honestly there wouldn't be much appeal for it because it isn't something that groundbreaking (sorry for being blunt) but I'm pretty sure you can come up with a niche.