I often have an idea, or hear an idea, of the form: it would be great if we could do X. But X has some small part x that is much much easier to do than X, and gets 80% of the value.
For example, autocomplete. It would be great if an editor could suggest semantically valid tokens at the cursor position. However, this is hard, because knowing what is semantically valid often requires, among other things, building an abstract syntax tree of the language in use.
A much easier thing is suggesting some possible tokens based on keywords in the language or tokens that appear elsewhere in the file. And I find it about 80% as useful as autocomplete. The sublime text editor does this:
Another example I've heard is getting a computer to drive a car autonomously. This is hard (though google seems close). A far easier thing might be getting a computer to drive a car in particular situations, like stop-and-go traffic on the freeway. I could read a book while my car inches forward when there's space available, and stops when there isn't.