I posted this on the Victory Conditions thread;
It is possible to highlight the endgame with a societal decline. In the last turns/time hours before the finish line there could be trade stagnation, natural disasters escalating, and barbarian migrations.
Your city should be stacked with debuffs and expected to endure and outlast the timer.
That would make the endgame transition more apparent and also leave room for improvement post endgame. Instead of having maxed everything out and having only stagnation, a post endgame city will be able to clawback development and technology with the player having an open padded timeframe in which they can end the game themselves.
I think struggling to overcome the Bronze age catastrophe would make for an excellent ‘boss battle’ finale for an epic campaign starting in the neolithic.