Biden has been erratic, not the gaffe-prone Biden we remember— that Biden was undisciplined, diplomatic and sharp. This Biden is unpredictable and disturbing. He spins his wheels, meanders onto tangents, and stumbles over simple points. When he interacts with people, he veers from uncomfortably familiar to aggressive.
Trump will say that Biden is senile and unfit for office. He won't just let attack dogs say it like other Republicans— he will say it explicitly and repeatedly, a major part of his campaign. He is not going to care that one might say this about him because he has the sociopath's shamelessness and audacity. He will be cruel and bullying about it, with the benefit of low expectations because no one expects anything else. This will be brutal in debates, because for all of his faults, Trump is still quick on his feet verbally and very conscious of this advantage.