Industrial Revolution 3 - it provides a very different experience and more depth while not blowing the complexity completely out of proportion. Industrial Revolution 2 is also still fun, and may be preferable if the addition of more complexity in the oil processing that 3 did is not your thing.

If you’ve been playing factorio vanilla for now I recommend Krastorio 2. Make sure to also install the recommended companion mods from that page. Krastorio is pretty much as easy to understand as vanilla but gives the whole game a different vibe, has some cooler end game gear and is more complex in some ways and easier in others. It’s my favorite overhaul mod so far, but also the one that I’ve played most.

Space Exploration apparently takes like 200 hours to complete, which I find daunting. But I have started it and I think I like it, so far, but I haven’t gotten to space yet ^^

If you’re in for a different kind of challenge I can recommend warptorio as well, which has vanilla tech, but you end up on a different planet every 20 min - hour, and research unlocks a bigger platform etc. Make sure to always warp when the first biters come, you’ll lose otherwise. Only in late game with flamethrowers it’s not that dangerous. This mod is so cool and you will have so much spaghetti in the lower levels of your warp platform ^^

I enjoy the Omnimods, they change all resources you can extract for the same omniore you need to process into the rest of resources.

Also, to give it a twist I like to add dangOreus which is a map where you are surrounded by resources but you can’t step on them! The ores closer to the center end quickly and you need to rebuild slowly until you can get to a farther place where they last longer.

It’s not as complex as other suggestions, but it’s fun!