I see several scenarios. Perhaps others can correct any misunderstandings I might have, or can suggest other alternatives...?
1. He will be defeated in 2020 and, because there will be a President elected who will not be sympathetic to his plight, Trump will be indicted for, amongst others, his obstruction of justice.
2. Fearing the likelihood of #1, he will step down before the 2020 election, in the understanding that President Pence would do a Gerald Ford and pardon him of all crimes, past and future.
3. He will be impeached, with the same outcome as #2.
4. He will lose the 2020 election and will then step down before Jan 2021, to allow Pence to briefly take the presidency in order to pardon him (not sure of this one...is that how it would work?)
5. He will be re-elected in 2020, allowing him to escape prosecution, as the Statute of Limitations will expire before his second term concludes.