When quilters say they are making an heirloom quilt, I think all they really mean is that they are spending extra time and care on it, more than they usually do when making a quilt. Time spent, quality, pattern, color, amount of use, none of that has anything at all to do with whether or not the quilt will be an heirloom. It's not under the control of the quilter.
Quilts become heirlooms, they are not made as heirlooms. The idea that there is some special way to make a quilt that will somehow set it on the path to "heirloomnicity" is not realistic. It's no more possible than setting out to build an antique table. Only time and provenance make heirlooms, not the human hand.