I was writing a blog about writer's prompts and realized that they served much the same purpose as block-amonth type projects: a challenge to try something new or different, to learn a technique or get better at a known one without committing to a whole book/quilt. That it results in a decent sized quilt once all the blocks are made is bonus. (They can also be turned into place mats, bags, wall hangings, etc., and often quilters never quite get around to doing anything with their results, just like writers, who rarely publish their prompt-based "practice" pieces without major modification.)
Along that line, how about a block-a-month prompt: not a specific block (which I have yet to figure out how to post online even when I've developed instructions), but a category or suggestion for something to look up or try from our collection of books, magazines, and free patterns online. Just to show how it works, I'll start with something easy: half-square triangles. Find a block that calls for half-square triangles and make it.
You can do the same size or different sizes each month (or however rapidly I post prompts that you feel like making). You can make matching fabrics or use scraps (the idea is practice, and you can make a nice comfy quilt for yourself even if you don't really like the results, and different sizes can be assembled into a single quilt though it helps if you do 4-8-12 (pus seam allowance on each size), 3-6-9-12 (plus seam allowance on each size) sequential sizes for easier assembly without borders). You can make more than one if you find several squares in the category that look fun.
If you are all too familiar with half-square triangles, look for a different way to make them (I've found at least three, all called the same thing) or invent your own block that uses at least four triangles.