Learn Spanish (Latin America): Ya salvaré la ciudad cuando termine esta siesta. - I'll save the city later, when I finish this nap.

Normally, yes! But when you pair ya with the future tense (salvaré), it changes meaning. This is called 'pragmatic displacement.' It pushes the action into the indefinite future, often to reassure someone or brush off a task. Here, it translates closer to 'I will get around to saving the city later.'

Could I just leave ya out and say Salvaré la ciudad cuando termine esta siesta?

You could, but you would lose the playful, dismissive tone. Without ya, the sentence is just a neutral statement about your schedule. Adding ya adds the flavor of 'Don't worry about it now, I will handle it later.'

In Latin America, is it natural to use the future tense salvaré instead of voy a salvar here?

Yes! While Latin American Spanish strongly prefers ir a + infinitive for most future actions, the formula ya + future tense is a major exception. Fixed expressions of reassurance or delay, like ya veremos (we will see) or ya lo haré (I will do it later), very frequently use the one-word future tense.

Why is termine in the subjunctive here, rather than the indicative termino?