Hmm. Without knowing (in some detail) what you already DO know, it's almost impossible to suggest something challenging you may not know.
Also, some people have multiple years of increasingly in-depth experience. Some people have the same shallow experience repeatedly over multiple years. Which description would you say best fits this question?
After more than 25 years of experience, I've learned a lot but also forgotten a lot. These days, though, I tend to find enough challenges just looking at questions posed by people on forums like this one and others and wondering if there's a good way to address them.
Maybe that's a way you could find the challenges you seek. See if you can answer questions here and on other forums both accurately and appropriately.
The problem is that Access is a tool. How much can you learn about a tool? OK, depends on the tool, but it is limited. But there is the issue of how many techniques have folks devised for USING that tool? The issue is not that once you know Access, you know databases. Because you don't. There are as many different ideas about databases are there are databases in the world. You are looking at trees and not seeing the forest.
Since what I'm about to say is correctable, don't take it as an insult, but rather as a motivation to open your mind... Your problem is that your imagination is what is limiting you. You believe that the limit of flexibility is built into Access - but it is really built into you. Let your imagination loose to see that the REAL challenge isn't what Access can do - but what YOU can do.
That's easy, do something that some one says you can't or shouldn't do with Access.
Convert access into some other tool: a text editor, an image editor, a spreadsheet, wysiwyg html editor, make it your email client, turn it into a media center that actually plays videos and music,
This demo is in response to a recent thread on creating "custom controls" https://www.access-programmers.co.uk/forums/threads/is-it-possible-to-create-custom-controls.320996/ Unlike other development environments access does not have a way to make a user defined or custom control. However, but...
A golf score programme to track scores per hole at different courses, taking into account stroke index, and golf handicap.
A sports programme to record match results for a given league and sort the teams into appropriate ranking order.
A genealogy app to manipulate a family tree.
That's 3 ideas that are interesting enough to be worth doing, and difficult enough to be a decent learning project without being too complex. Is that the sort of things you mean?
If you want to make a difference, pick up where Stephen Lebans left off http://www.lebans.com/. That guy knew his way around the internals of Access like nobody else. God only knows where he got the information to build the stuff he did, most of which remains functional to this day. I've used his picture class in several things, and I've got my heart in my mouth that one day, MS will change something inside and it will stop working. I have no idea what I would do if that happened. Nobody else I know of makes anything even remotely similar.