I mean, shipping and OCs and theories are super fun and things I definitely encourage!! But honestly..... what I hoped for from the very start was for people to care about these characters. Not in a "I WANT TO MAKE PEOPLE CRY B/C THEY FEEL FOR THEM" sort of way, just... having opinions about these people, as people. Relating to them or disagreeing with them or liking them or not trusting them.
I still remember being very excited when, as a character was introduced by bodily throwing the protagonists out of his home, someone responded to the page with "nevermind, that guy's an ass!" It's weird, it's such a small thing, but it feels like these characters could never have become real until people reacted to them like that.
Make sure you tell people that you're open to this!! Crit without being asked for it can honestly be kind of rude, and not all artists react well to it, so a lot of folks will avoid it unless they know it's wanted.
I don't really like unsolicited crit, so something that worked really well for me was putting out a specific Call For Critique to my readers after my first chapter was finished, as basically a "now that you've seen enough of the story to get a feel for it, what can I improve?" type thing. I got a lot of good and really helpful responses from that!