I've been working on a project that is something like "a framework of abstractions". I've found that studying and developing its concepts has been a great exercise in abstract thinking. Check it out and see if it's the type of thing you might be interested in. https://github.com/perspectivesonatheme/patterns

I'd also recommend learning some Haskell and category theory. It's difficult stuff and I won't pretend to have a solid handle on either, but what I have learned has been eye-opening.