A few more ideas just kind of being thrown out in quick succession.

I think DAILY QUESTS could be overhauled a bit. Instead of having the chance to win something daily if you do a thing, it would be neat if there were new little objectives that you could complete daily. You could also have a collection of different quest items you could get but the one you get is random, sort of like how stickers work.

You could likely have a WEEKLY QUEST, which would work the same as daily except it's weekly. You could also have quests that aren't tied to any particular time constraints.

While I do love the idea of sequential quests, where you have to complete one quest before you can complete the other, I think it would be really neat if you could do the same sort of thing but in any order. Maybe the system for how that type of quest works could be designed so you could do either one with the same system.

I just wanted to say I really like the idea of quest items having descriptions. I also think it should be easy to get a really zoomed in picture of them, and download them to take them into an image editor program or something like that. It would be fun to hide things in those kind of places.

Quest items could also just be a unique brand of stickers if you wanted, but I'd really like to see them be a little something more.

The idea of QUES ITEMS isn't all too different from achievements. I'm pretty sure Game Jolt Client has something along those lines, but I haven't used it yet myself so I can't comment too much on that.

Hope you find these ideas helpful.