What if stuff had been invented earlier than it was?
Part of the fun here is that, rather frequently, it has been invented earlier. And we learn the importance of cultural settings. Because the Greeks invented a steam engine and used it as a toy. And many ancient civilizations had the waterwheel, but the medieval Europeans went berserk, thinking of all the ways they could use this wonderful, wonderful, wonderful thing all over the place. And for all the talk of the breakdown of the Roman Empire, it was where that breakdown was most total that we see them going crazy over the waterwheel. The Byzantine empire didn't think much of it.
Guns, for instance. That can be complicated. Earlier guns were worse weapons than the crossbow. But the soldiers could be drilled in them more easily. The first use involved pikeman to protect the musketeers. Which was inspired by certain Roman legion tactics, so perhaps it could have been used earlier.
Printing press would have been fun in China. You had too many characters to make it practical.