A smart candidate will know that getting into the mud with Trump will only result in moving the debate into an area that Trump is comfortable with. Where he isn't comfortable is in deep policy dives where he has to compete with someone who genuinely knows the numbers and can contextualize them in a way that average voters can digest. Talking about Trump's accusers, Stormy Daniels etc. would only backfire. It could be briefly glossed over, but whoever runs against Trump should be focused on policy and tangible ways their platform will help the country. The negativity that Trump brings to the table will have already been on full display for 5 years by then, so there's no use in wasting debate time over it. Trump and his base rely on the oxygen generated by negativity and so a distinctly positive candidate would deprive them of it.