What would have happened if Julia turned out to actually be a Julius, and that Julius survived to adulthood?
Five things come to mind:
1. Julius II is unlikely to marry Pompey

-- so those alliances get altered somewhat.
2. For some reason, I don't see Caesar (assuming he gets the same power he did IRL) adopting Octavian if his son is still alive.
3. Cleopatra might find it more difficult to mess around with Caesar since Caesar now has a legitimate heir.
4. I can see Julius II either serving under Pompey (in lieu of the marriage) or serving with his father's forces in Gaul and places like that.
5. There is no way Julius II is going to be appointed emperor since it's obvious the Senate saw Caesar as a special case and the Republic was still going on. Sure, they have made Julius Caesar perpetual dictator, but interpreting it to cross generations is a bit of a stretch
I'm thinking of two scenarios:
1. Julius II has Julia's lifespan and dies in 54 BC.
2. Julius II lives to the age of 65 (I was going to say have his father's lifespan, but that ended rather abruptly) and dies in 18 BC (assuming no one kills him first).
I have an idea of organizing a communal timeline here with this POD -- I don't know enough about the early Roman Empire to do it myself.