The sky's the limit
There is no apparent limit.
Some sources claim that 'the sky's the limit' was coined by Cervantes in Don Quixote. This appears to add to the list of popular fallacies about coinages attributed to Cervantes; for example, 'wild goose chase' and 'don't put all your eggs in one basket'. These phrases were introduced in early translations into English of Don Quixote which are now regarded by scholars as loose paraphrases of the original. None of these phrases appear in Cervantes' original text.
The latter two phrases given above are both quite old, i.e. 17th century. 'The sky's the limit' is much more recent. It originated at a time of optimism and progress - in the USA just before WWI. The earliest citation I can find is from the New York newspaper The Syracuse Herald, September 1911:
"Then good luck, and remember the sky's the limit."
A slightly earlier, modified version of the phrase appeared in the US newspaper The Stevens Point Daily Journal, in August 1899:
"When the papers were arranged, the players got a new deck of cards, and there was not a word passed while the cards were being chuffled. The sky was to be the limit until the $50,000 was reached."
The adoption of the expression was no doubt influenced by the invention of the aeroplane. The phrase was picked up and used as the title of a Fred Astaire/Joan Leslie film at another time of intense interest in powered flight, in the middle of WWII - 1943.