Automation and Factory Builder Games

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've always been so interested in the overhauls but I've had factorio for 7 years at this point and I still haven't beaten it on vanilla so I can only imagine how screwed I'd be on an overhaul

Pyanodon is very very very complicated, but fun in a different way. You might spend 10 hours just automating one recipe. You also probably would want to add some quality of life mods like Far Reach, Early Trains, Jetpack, and of course FNEI so you can look up recipes. Py is less focused on optimal factory designs and more about many different cascading product types.

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.

I have always been a huge fan of AngelBob. It allows your production to scale so much and it is really nice to have different methods of extracing minerals from Bob ores. Yes, in the midgame you will need big storage facilities, but in the endgame you will realise that everything evens out. My city block AB game with a goal of 2700 science/min really made me appreciate all of the balancing. The only complaint that I always had was that Bob modules are too powerfull (especially god modules). So I would recommend people to stop at lvl2-3.

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!