My opponent casts Promising Future, banishing Elder Dragon. I banish Fizz. At which point do I choose the spell Fizz replays? When this happens, can I target his Elder with a Star-Crossed replayed from Fizz?

Wrong order of execution

You pick the trash spell when Fizz's "when you play me" ability is finalized onto the chain — not when Fizz hits the board — and by that point Elder Dragon is already in play, so Star-Crossed can legally choose it. Here's the flow. And remember, finalize up the chain, resolve down the chain. Your Fizz enters pending first. Opponent's Dragon enters pending right after. Fizz finalizes and resolves, Fizz' 'When you play me' goes as a pending item on the chain. Dragon finalizes and resolves, Dragon's 'When you play me' goes as a pending item on the chain. Fizz' ability finalizes -> choice of spell is made here. Dragon's ability finalizes -> targets are chosen. Dragon's ability resolves. Fizz's ability resolves -> the chosen spell becomes a pending chain item. Finalize the chosen spell -> pick it's targets. Resolve the spell. It might very well be that Fizz is already dead by the time your Star-Crossed resolves. But during finalization of Star-Crossed you can pick targets. You need 1 friendly and 1 enemy unit for legal target allocation.