As title goes, I'd love to see bigger Mastery Challenges—12q 6s, or 30q 15s, etc. Helps students master big courses like the Math ones, with over 10000 Mastery Points. Sure, Mastery Challenges allow for spaced repetition and spiral review, but they're so fun that I'd like to test more skills per Challenge: 3 skills for a HUGE course is a tad inadequate, given the long 12-hour wait time.
But even if I can use Unit Tests and Course Challenges, Mastery Challenges are unique and fun, because they appear and disappear, and give that feeling of, "Yes! I've mastered this!" that Missions did a fantastic job of. Minor Missions replacement in Course Mastery, in a way!
I'd also like to see a Math domain Mastery Challenge on the Math landing page: same Mastery Challenge, but one that pulls from courses you've studied, much like the World of Math challenge!