Sorry, this is not the right problem in financial services. Companies like Vanguard are already doing a great job of this and the costs are extremely low. Its a commodity product with razor thin margins that actually serves the needs of its customers well. Maybe there's a marketing issue where they aren't educating enough people, but that's not a technology problem.
As an alternative: Lower the Costs to IPO, disrupt Investment Banks
Sarbanes-Oxley, minimal competition between investment banks, and heightened SEC scrutiny have made the fixed costs to an IPO astronomical. These days a company, for the most part, cannot IPO for less than a $1 billion raise. This means that the broad public, including those index funds YC loves, is prevented from enjoying any returns at all for younger, high-growth companies.
There is room for startups to disrupt part or all of the process. It would be capital intensive and hard as hell. But, you're not looking for easy right?