What's amazing is that what seems hard to you now will seem easy after you get more practice. Keep on stretching your boundaries and don't make the same quilt over and over. You're smart to want something new to do that lets you practice the 1/4" seam.
I agree with others who have suggested French Braid. It gives tremendous bang for the buck and really makes you think about fabric selection. OBW is another easy-to-make quilt that gets a lot of attention.