That would be the Speaker chosen by the just-elected Congress after the election. A republican majority Congress would choose which Congressional Representative???
Remember that MAGA is not the Trump Plan
He was chosen by the Military to head the MAGA plan.
If another Republican is chosen to be President, Trump can still serve in some capacity.
Think of how Putin swapped jobs with his Prime Minister, so when Georgia attacked Russian peacekeepers in 2008 it was President Medvedev that ordered the Red Army into Georgia, not Prime Minister Putin.