An app for U.S. investors where a user entered asset positions and % gains and suggested ETFs that best diversified the portfolio would be useful. It's very hard to beat the market "from scratch" on pre-tax basis, but many investors have undiversified taxable portfolios that could be improved through proper diversification with uninvested cash.

In my case, I have some positions in a taxable account with capital gains exceeding 300%, such as XLK (tech stocks) and an S&P 500 index fund. I wish I had less in tech, but selling XLK would incur long-term capital gains taxes. For diversification, I could invest new money in ETFs that are less correlated to tech. An app would suggest which ones and with what weights.