Designing is a tricky business. Many times the issues faced include those along the lines of having too many designs in one’s head and not enough time in the day/week/life to knit them all. Others include grading for various sizes, arithmetic (you’d be surprised at exactly how much math is used in designing, really), yarn or stitch pattern behaviors conflicting with intentions.
Much of my current designing dilemmas surround a finite amount of yarn. I have only so much yarn but so many ideas for it.
A skirt would be great out of this linen! Or a summery top! Or some curtains! Or a lacy wrap!
This wool would be perfect for a long sleeved pullover! A cabled cardi would be oh-so-cute though, not to mention a matching hat/scarf/mitts set!
Sometimes, designers just have to bite the bullet and pick their most inspirational idea, or their most practical. Sometimes we have to flip a few coins, roll a few dice.
How do you decide what to create when there are a plethora of options and only so much yarn?
SillyLittleLady's Space 
Pretty pretty yarn! No wonder you have so many ideas for it.
Sometimes, actually, if I can’t decide which one to do, the way I decide is to cast on and knit a swatch of all the different pattern designs, and let the yarn decide. Usually that method means there’s an awful lot of frogging going on, but too, it usually means I’m totally happy with what gets knit.
If I have more than one design in mind for some yarn, I usually just buy more yarn. So possibly I’m not the best person to answer. : ) Course, I do usually use yarn recycled from thrift store sweaters, but still. That or I put those particular design on the back burner until something speaks to me more definitively.