The best indicator you could probably develop yourself would be a tool to accurately classify market conditions for any current pair. If trending, trend trade, if ranging trade the ranges. The indicators can be useful but they do not tell you anything you can't already see in the charts. RSI is usually good for ranging markets and when the market is ranging and you're trading with RSI signals, you will think it is the most incredible tool.
But when market conditions change, then the RSI is now completely pointless which means that that period of good predictive behaviour was just a coincidence to the market conditions having been favourable to that style of trading.
Look at how pointless this Stochastic Oscillator is as an example, where standard oversold (buy signals) consistently produced only the most pitiful bullish price moves, whereas the market has been in a consistent downtrend.