See comparison on the chart - practically a mirror image of each other.
Both have a Head and Shoulders pattern long-term, Ethereum's is the inverse of BTC Dominance. BTC Dominance may drop below 50% to the thick yellow line, Ethereum's BTC value may rise sharply to its thick yellow line. Potential for movement beyond those lines is possible.
Remember that USD value may not reflect this in the same way - BTC and Ethereum USD value may go up or down as Ethereum's BTC value rises. That said, I believe Eth's USD value will rise with a significant BTC value change like this.