I honestly don't have any of those type of regrets. I am grateful that my sons wore all of their t-shirts out (and are still wearing some) and never had any interest in a t-shirt quilt. I just don't like them and I know I would have hated making them.
I do regret buying fabric for curtains for every room in the house we built. After we moved in (when I planned to make them) I realized we live far enough back and had enough porches that our house just doesn't need them. It is a log home and the scenery makes it work. Now, I have yards and yards of fabric from the early 90s. It frequently appears on the backs of projects!