We know that 6v6 has its own set of issues opposed to the issues of 5v5. Swapping systems only swaps your problems, which means it a pick your poison situation. But it wont solve the fundamental issues.
The fundamental issue is that tank play isnt as fun as dps play while tanks are necessity to make the gameplay work.
The game would be the most fun if everybody could play what they want to play without being reliant on what your teammates run.
So my idea would be to redesign the game to make every player more selfsufficent.
Maybe we could have deplloyable items on the map like healthpacks which help to fulfill roles which might not be covered by teammates.
What if you could pick up a one time use deployable barrier? Or other defensive mean to hold the frontline? Then the necessity for a tank player would lessen.