From Trump, we will get lies and nonstop aggression, because, true to form, he believes that the aggressor will be seen as the winner regardless of the substance of the discussion. In some cases, there is some merit to that, but not this one; obnoxious behavior is unlikely to sell him to the suburban housewives he is trying desperately to win over.
From Biden, we will see pleas for decency and unity and lots of counterpunching. Most of the memorable lines from past debates were counterpunches, so this is an advantage for him. Be ready with those zingers, because there will be plenty of opportunities to use them.
Both candidates will reinforce the public perception of their respective personas. As a result, nothing will change.