Where do I begin?

[RippleGylf she/her]

I've read all of the published Cosmere books, and I want to dig deeper into the main theories. Where should I start?

[Pagerunner he/him]

Have you checked out the Theoryland interview database? It's primarily a Wheel of Time site, but they do still update with Brandon's signings, so they've got a lot of juicy tidbits that were obtained at signings that you can't find anywhere else.

If you look in peoples' sigs, you'll see some of us have an "Espoused Theories" section. If they're following the suggested format, anything without an asterisk would be someone else's theory that they like. If you see a theory popping up in multiple peoples' sigs, you know it's a good one.

You could use the search bar to filter by forum (Cosmere Theories has more overarching ideas, while for now Edgedancer/Secret History spoiler forums tend to be the most cutting-edge about those topics) and search for a topic you're interested in.

But, probably, just start clicking away through a forum. If you find someone referencing a theory that you're not familiar with, quote them and ask for more info. I doubt anyone will complain about being 'forced' to talk some more about an idea of theirs. Posting theories of your own usually brings us out of the woodwork, saying "Cool idea, I think that fits with my theory, too."

[A Budgie she/her]

Hm...really, I would recomend just going through the different topics, on the theory boards and book ones, and picking out the ones you want to know more about. If you have a direct question, I would think just asking is probably the best way, or doing a search to find some information.
Or, you could go on the Coppminds, which has both given information from the books and a certain amount of theory.

[Itchy Savant he/him]

Any board on here! comers 102 is probs a good start, and then from there the cosmere theories board is probably where you will find the bulk of the stuff. There are other theories in the other boards as well but they are more specific generally

[Jofwu he/him]

There's not really a place where all of the most popular theories have been collected. You just have to stick around for a while before you start to soak them in. Pull up the forums for the books that interest you most and skim through for an interesting topic. Or look for things that have lots of replies.

You can also join us on Reddit. The most active subreddits are /r/brandonsanderson, /r/cosmere, /r/Stormlight_Archive, and /r/Mistborn.

[The One Who Connects he/him]

I'm going to out myself here (to the few who understand it) and say that what I did was explore everything. I went to the furthest back page of theories in each subforum and worked from there back to the present. If something looked interesting, I checked it out, and if not, I kept going. You'll find all manner of little facts and things which'll make you think a bit differently about stuff later.

All else fails, just make a thread with your ideas and post it, kinda like you did here.